Brs 


DEFT 


lies 

et, 


ROASTING 


OF 


GOLD  AND  SILVER  ORES, 


AND    THE 


Extraction  of  their  Respective  Metals 

WITHOUT  QUICKSILVER. 


BY  G.   KUSTEL, 

MINING  ENGINEER  AND  METALLURGIST, 

Author  of    '-Nevada  and  California  Processes  of  Silver  and  Gold  Extraction,"  and 
"  Concentration  of  all  Kinds  of  Ores." 


NEW     EDITION. 

ILLUSTRATED  WITH  NUMEROUS  ENGRAVINGS. 


A.  J.  LEARY,  STATIONER  AND  PRINTER,  402  &  404  SANSOME  ST. 
SAN  FRANCISCO,  1880. 


Entered  according  to  Act  of  Congress,  in  the  year  1880, 

BY  G.  KUSTEL, 
In  the  Office  of  the  Librarian  of  Congress,  at  Washington,  D.  C. 


PREFACE  TO  THE  SECOND  EDITION. 


The  favor  with  which  the  first  edition  of  the  present  treatise 
on  Roasting  of  Ores  was  received  by  the  mining  public,  and  the 
increased  inquiries  after  a  new  edition,  induced  the  author  to  write 
the  present  book.  During  the  long  interval  of  ten  years  which 
elapsed  since  the  first  edition,  many  improvements  in  the  construc- 
tion of  furnaces,  as  well  as  in  the  way  of  roasting,  have  been 
introduced;  this  edition,  therefore,  has  undergone  many  alterations, 
and  new  cuts  of  furnaces,  etc.,  have  been,  added. 

San  Francisco,  Feb.,  1880,  t   G.  K, 


390321 


PREFACE  TO  THE  FIRST  EDITION. 

The  publication  of  this  Treatise  is  due  solely  to  the  many  inquiries 
concerning  the  "Leaching,  Solving  and  Precipitation  Process  for  Silver 
Ores,"  now  successfully  practiced  in  Sonora,  Mexico,  where  it  has  been 
lately  introduced  by  Mr.  Ottocar  Hofmann. 

In  consideration  of   the  very  important  preparation  of   the  ore, 

before  it  is  subjected  to  the  Solving  Process, — namely,  the  Roasting, 

I  have  thought  it  proper  to  devote  considerable  space  to  the  descrip- 
tion of  different  modifications  of  this  operation,  which  is  regulated  by 
the  peculiarity  of  the  ore,  and  by  the  subsequent  treatment.  It  is 
impossible  to  give  any  one  way  which  will  be  suitable  in  every  case; 
for  this  reason,  and  in  order  to  cover  all  cases  as  far  as  possible, 
a  detailed  description  of  different  modes  of  Roasting  will  not  be 
superfluous. 

The  Solving  Process,  as  now  practiced,  is  a  very  economical  method 
for  the  extraction  of  silver,  for  the  reason  that  no  quicksilver  and  no 
castings  are  used  except  what  are  needed  for  crushing.  Mills  in 
Mexico  being  dependent  on  San  Francisco  for  the  shoes,  dies,  gearing, 
etc.,  of  amalgamating  pans,  millmen  there  know  how  to  appreciate  a 
process  confined  to  wooden  tubs  requiring  no  power.  A  comparatively 
small  capital  is  necessary  for  building  up  such  works,  and  hence  there 
is  a  more  reasonable  ratio  established  between  the  amount  of  money 
which  must  be  expended  on  the  works'  and  the  real  value  of  the  mine, 
than  where  other  more  expensive  machinery  is  employed — a  circum- 
stance which,  being  insufficiently  regarded,  is  often  the  source  of 
failure. 


4  PREFACE. 

Mr.  O.  Hofmann  commenced  first  with  the  " Chlorination  Process" 
but  finding  great  difficulty  in  obtaining  the  regular  supply  of 
sulphuric  acid  and  manganese  from  San  Francisco,  abandoned  the 
chlorination  with  cold  chlorine  gas,  which  is  indispensable  in  the 
presence  of  gold.  Another  difficulty  was  in  obtaining  a  good  article 
of  sulphide  of  sodium.  He  tried  to  extract  the  potash  from  ashes, 
and  to  use  this  in  place  of  soda,  but  decided  finally  in  favor  of  lime, 
which  is  found  in  abundance.  From  this  the  sulphide  of  calcium  is 
easily  manufactured  on  the  spot.  Sulphide  of  calcium  was  first 
applied  by  Kiss. 

The  Solving  Process  is  very  simple,  and  readily  performed  by 
common  workmen;  besides  the  lime,  only  brimstone  must  be  provided, 
in  order  to  prepare  the  necessary  chemicals  for  solving  and  precipita- 
tion. It  is  a  general,  but  erroneous,  belief,  that  the  solving  is  a  slow 
process.  An  amalgamating  pan  is  charged  with  500  to  1,000  pounds 
of  roasted  ore,  and  treated  at  least  six  hours,  and  therefore  turns  out 
at  most  two  tons  in  24  hours ;  while  a  box  or  vat  of  proper  size  used 
in  the  Solving  Process,  can  work  from  four  to  ten  tons  in  the  same 
time. 

Only  those  ores  are  treated  by  this  process  which  absolutely  require 
roasting;  which,  however,  with  improved  furnaces,  is  not  so  expensive 
as  it  used  to  be.  The  chloride  ores  alone  can  be  leached  directly 
without  roasting,  and  this  when  there  is  no  other  silver  combination 
in  them. 

G.    KUSTEL. 

MARCH,  1870. 


I 

Page. 

Introduction 7 

Important  Silver  Ores 7 

Free  Milling  Ores 9 

Difference  between  Silver  and  Argentiferous  Ores 9 

Important  Combinations 10 

Means  of  Desulphurization 10 

Result  of  Desulphurization 12 

Means  of  Deoxidation 13 

Desulphurization  not  Efficient 14 

What  a  Chloride  is,  and  how  Chlorination  Effected 14 

Chlorine  Gas 14 

Means  of  Separating  the  Metal  from  Chlorine 16 

II. 

Roasting  of  Ores '. 18 

A.  Chloridizing  Roasting 21 

When  Salt  should  be  Introduced 22 

Mixing  of  Salt  and  Ore 23 

Sulphate  of  Lead 25 

Necessary  Amount  of  Sulphurets . 26 

Amount  of  Salt  to  be  used 28 

Condition  of  Metals  after  Roasting 30 

Permanent  Stirring  not  Essential 31 

Signs  of  Good  Chloridizing  Roasting 32 

Chlorination  Assay 32 

To  Regulate  a  Chloridizing  Roasting 34 

Means  of  Destroying  Base  Metal  Chlorides 36 

Steam  Decomposes  Base  Metal  Chlorides 38 

Application  of  Steam  in  Roasting 83 

Chloridizing  Roasting  of  Silver  Ore  containing  Lead 39 

Difference  in  Roasting  Ores  for  Pan  Amalgamation  as  compared  with  other 

Modes  of  Extraction 40 

Roasting  Charges 40 

Roasting  of  Silver  Ores  containing  Gold  and  Copper,  to  be  treated  by  Chlo- 
rine Gas  and  Leaching '. 40 

Roasting  of  Silver  Ore  in  Freiberg 43 

Roasting  of  Argentiferous  Copper  Ore  at  Arivaca,  (Arizona) 44 

Roasting  of  Copper  Matte 45 

Roasting  of  Black  Copper 47 

Roasting  for  Augustin's  Process 49 

Roasting  of  Silver  Ore  in  Long  Furnace 50 

B.  Oxidizing  Roasting 52 

What  Process  requires  Oxidizing  Roasting 53 

Roasting  of  Copper  Matte  53 

Roasting  of  Iron  Pyrites  containing  Gold  and  Tellurides  of  Gold 56 


INDEX.  — CONTINUED; 
III. 

Roasting  Furnaces ;;;...;...  6$ 

Reverberatory  Furnaces 65 

Single  Roasting  Furnace 65 

Double  Roasting  Furnace 69 

Muffle  Furnaces 74 

Bruckner's  Revolving  Furnace 75 

Modified  Bruckner's  Furnace 76 

Dodge's  Revolving  Furnace 77 

Revolving  Hearth  Furnace 78 

Burton's  Revolving  Furnace 79 

Park's  Roasting  Furnace 81 

Continuous  Feeding  Furnaces 82 

Howel's  Improved  White  Furnace 82 

O'Harra's  Mechanical  Furnace 84 

Stetefeld's  Roasting  Furnace 88 

IV. 

Lixiviation 103 

Extraction  of  Silver 104 

Leaching  Vat 105 

Precipitation  of  Silver A 113 

Precipitating  Vat 114 

Treatment  of  the  Precipitated  Silver 116 

Applicability  of  the  Lixiviation  Process 117 

Hyposulphite  of  Soda 119 

Hyposulphite  of  Lime .' 120 

Sulphide  of  Calcium 122 

Ottokar  Hofman's  Process 122 

Patera's  Process 128 

Kiss  Process .' 129 

Patera  Roeszner  Process 130 

Augustin  Process 131 

Ziervogel  Process 132 

Extraction  of  Silver  by  Sulphuric  Acid 133 

V. 

Extraction  of  Gold  from  Sulphurets,  etc.,  by  Chlorination 136 

Extraction  in  a  small  scale 137 

Treatment  of  Sulphurets  by  Chlorination 140 

Application  of  Chlorine  Gas 142 

Leaching  Vat '. .  143 

Chlorine  Generator 146 

Ingredients  for  Generation  of  Clorine  Gas 148 

Lixiviation  of  Gold 150 

Precipitation 151 


I.    INTRODUCTION. 


Ores  are  classified:  a.  According  to  the  metal,  the  extraction  of 
which  is  principally  remunerative ;  as  silver  ores,  lead  ores,  copper 
ores,  etc.  b.  According  to  the  metallurgical  treatment ;  as  roasting 
ores,  smelting  ores,  milling  ores,  etc.  c.  According  to  the  predomi- 
nant gangue;  as  calcareous  ores,  quartzose  or  ochery  ores.  d. 
According  to  the  predominant  metallic  mineral;  as  sulphuret  ores, 
chloride  ores,  carbonate  ores,  etc. 

I  mportant  Silver  Ores. 

The  most  important  silver  ores  are  those  found  in  such  quantities 
as  to  be  an  object  of  metallurgical  operations.  The  principal  min- 
erals of  this  kind  are  the  following: 

A.  Silver  Ores  with  unvariable  amount  of  Silver,  a.  Sulphuret  of 
Silver,  or  silver  glance,  with  87  per  cent,  of  silver.  It  is  of  common 
occurrence,  and  is  the  most  suitable  of  the  silver  sulphurets  for  pan 
amalgamation  without  roasting,  b.  Brittle  Silver  Ore,  (Stephanite) 
or  sulphuret  of  silver  and  antimony.  This  mineral  contains  68  per 
cent,  of  silver,  and  is  quite  common.  c.  Polybasite,  sulphuret  of 
silver,  antimony  and  some  arsenic,  with  75  per  cent,  of  silver, 
Horn  Silver,  or  chloride  of  silver,  with  75  per  cent,  of  silver,  occurred 
massive  in  White  Pine,  Nevada,  Arizona;  iodic  and  bromic  silver 
of  yellow  and  green  color,  d.  Ruby  Silver.  The  dark  red  silver 
ore,  or  antimonial  variety,  with  59  per  cent.,  and  the  light  red  silver 
ore,  or  arsenical  variety,  with  65  per  cent,  of  silver,  are  valuable 
minerals,  They  occur  quite  frequently  in  Nevada,  Idaho,  Montana, 


8  INTRODUCTION. 

Mexico,  etc.      e.  Miargyrite,  sulphuret  of  silver  and  antimony;  36.5 
per  cent,  of  silver;  Idaho,  Montana. 

B.  Argentiferous    Ores,    with     variable    amount    of    silver;    as 
Stromeyerite,  or  silver  copper  glance,  a  sulphuret  of  silver  and  copper 
containing   up   to  53  per   cent,  of  silver;  beautiful   specimens  are 
found  in  the  Silver  King  and  Heinzeman  mines,  Arizona.  Stetefeldtite, 
with   25  per  cent,   of  silver,  is  an  oxide  ore  which  is  found  very 
frequently  in   Nevada  and  Arizona,     c.  Silverfahlore,  argentiferous 
gray  copper  ore.     It  contains  silver  in  very  variable  proportions  up 
to  31  per  cent.     This  ore  is  quite  common,  and  for  this  reason  is  im- 
portant.   It  is  also  one  of  the  most  rebellious  ores,  containing  copper, 
antimony,  arsenic,  sulphur,  lead,  iron,  zinc,  and  sometimes  gold  and 
quicksilver,     d.  Chloride   Ores,  (so  called)   mostly  decomposed  ores, 
generally  of  an  earthy  appearance  and  different  color.     They  contain 
more  or  less  finely  divided  chloride  of  silver. 

C.  Argentiferous  Lead  Ores,  galena,  or  sulphuret  of  lead,  lead 
glance.     Generally,  this  is  not  rich  in  silver,  containing  from  $20 
to  $60  per  ton.     Specimens  assay  sometimes  as  high  as  $300.     The 
fine-grained  variety  is  generally  considered  richer  than  the  coarse 
crystallized  kind,  but  this  has  not  been  observed  to  be  the  case  in 
Nevada  and  Arizona,     c.  Cerusite,  carbonate  of  lead.     If  pure,  with- 
out admixture  of  copper  and  other  carbonates,  it  is  poor  in  silver  in 
most  cases.     Raw,  it  amalgamates  only  too  readily  in  pans.     Smelt- 
ing is  the  only  proper  way  of  treating  galena  and  cerusite.   d.  Argen- 
tiferous  Zincblende,  sulphuret  of    zinc.     Pure    zincblende  contains 
usually  only  traces  of  silver ;  often,  however,  it  assays  well,  even  up 
to  $400  per  ton,  although  no  other  silver  ore  can  be  detected  with  it. 
In  some  mines  the  argentiferous  zincblende  prevails,  and  is  the  most 
important  ore.     It  requires  a  great  heat  in  roasting,     e.  Argentifer- 
ous Pyrites.     Copper  and  iron  pyrites  are  poor  in  silver,  but  often 
auriferous.     Pvrite  is  a  valuable  companion  for  silver  ores  which 


INTRODUCTION.  9 

have  to  be  treated  by  a  chloridizing  roasting,  on  account  of  its  amount 
of  sulphur,  which  is  necessary  for  the  decomposition  of  salt. 

There  are,  besides,  numerous  classes  of  decomposed  silver  ores, 
generally  of  earthy  nature ;  also,  half  decomposed  ores  -which  have 
lost  their  metallic  glance,  having  a  black  or  bluish-black  color,  and 
being  generally  cupriferous. 

Free   Milling  Ores. 

Free  milling  means  the  amalgamation  of  ores  in  pans  directly, 
without  roasting.  The  decomposition  of  the  ore  is  effected  by  the 
addition  of  chemicals  (blue  vitriol  and  salts).  Of  all  the  minerals 
above  stated,  only  the  silver  glance,  chloride  ores,  bromic  and  iodic 
ores,  are  free  milling  ores.  Less  favorable  results  by  free  milling  are 
obtained  from  stetefeldtite,  from  some  of  the  decomposed  ores,  from 
silver — copper  glance  (if  rich  in  silver),  and  from  stephanite.  All 
other  sulphureted  and  many  decomposed  ores,  especially  those  rich 
in  antimony  or  arsenic,  cannot  be  treated  in  pans,  or  by  lixiviation, 
without  a  proper  roasting. 

Difference  between   Real  Silver  Ores  and 
Argentiferous  Ores. 

Real  silver  ores  have  mostly  an  unvariable  amount  of  silver.  Real 
silver  minerals  admit  an  approximate  estimate  of  the  value  of  the  ore, 
if  the  proportion  of  ore  and  gangue  is  considered,  without  making  an 
assay.  Jbor  instance,  if  ruby  silver  ore  should  consist,  according  to 
guess,  of  one  part  of  ruby  and  about  three  parts  of  quartz,  or  some 
other  gangue,  then  one  ton  would  contain  J,  that  is,  500  Ibs.  of  ruby, 
which  always  assays  59  per  cent,  of  silver ;  therefore,  one  ton  of  such 
ore  would  be  estimated  as  containing  4,301  ounces  (troy)  of  silver. 
With  the  argentiferous  ores  it  is  different.  Fahl  ore,  for  instance, 


10  INTRODUCTION. 

may  be  very  poor  or  very  rich,  yet  its  value  can  be  ascertained  only 
by  an  assay.  There  are  no  means  of  estimating  the  richness  of  argen- 
tiferous ores  "by  sight." 

Important   Combination. 

With  the  exception  of  a  few  metal  oxides  of  iron,  zinc,  manganese, 
and,  among  silver  ores,  of  the  stetefeldtite,  etc.,  the  most  important, 
because  the  most  frequent  ores,  are  the  sulphureted  varieties.  Sul- 
phur is  the  most  formidable  obstacle  to  the  metallurgist  in  extracting 
metals  from  their  respective  ores.  Desulphurization  has  been  a  sub- 
ject of  most  diligent  and  numerous  experiments.  The  oldest  method 
is  the  application  of  heat,  which  is  still  in  use,  notwithstanding  the 
many  attempts  in  modern  times  to  dispense  entirely  with  fire  or  to 
modify  its  application  so  as  to  perform  the  process  more  perfectly  and 
in  a  shorter  time.  The  only  important  progress  in  desulpurization  by 
heat  in  a  very  short  time,  has  been  made  with  the  Stetefeldt  roast- 
ing furnace. 

Means  of  Desulphurization. 

The  desulphurization  of  ores  is  effected  :  a.  By  heating  with  free 
admission  of  air.  This  is  the  common  way  of  "roasting,"  and  the 
most  important,  and  is  effected  either  in  kilns,  heaps,  etc.,  or  in  re- 
verberatory  furnaces.  As  soon  as  the  sulphureted  ore  is  heated  to  a 
certain  degree,  one  part  of  the  sulphur  escapes  as  sulphurous  acid ; 
another  is  converted  into  sulphuric  acid.  Some  sulphurets  (iron 
pyrites)  lose  their  sulpher  without  the  application  of  heat,  being  de- 
composed by  exposure  to  the  action  of  air  for  a  long  time.  This  way 
is  sometimes  practiced  on  gold-bearing  pyrites,  b.  By  heating  with 
exclusion  of  air.  Only  the  sulphides  of  gold  and  platinum  are  de- 
composed perfectly  by  this  method.  Other  sulphureted  ores  lose  their 
sulphur  only  in  part,  being  reduced  to  a  lower  state  of  sulphide. 


INTRODUCTION.  11 

Sulphuret  of  silver  (Ag  S)  remains  undecomposed.  Cinnabar,  sul- 
phide of  antimony  (Sb  S3)  and  sulphide  of  arsenic  volatize  un- 
changed. Iron  pyrites  (Fe  S2)  gives  up  23  per  cent,  of  its  sulphur? 
being  reduced  to  magnetic  pyrites,  and,  by  a  strong  heat,  to  proto- 
sulphide  of  iron  (Fe  S),  not  further  reducible.  Also  sulphide  of  zinc 
(zincblende),  remains  undecomposed.  Copper  glance  retains  its  sul- 
phur, and  copper  pyrites  loses  only  one  part  of  the  sulphur  which  is 
combined  with  the  iron  in  it.  Galena  (Pb  S)  is  reduced  to  a  lower 
state  (Pb4  S),  a  part  of  the  lead  separating  out  in  a  metallic  state.  (Pb4) 
c.  By  superheated  steam.  Sulphurets  not  evolving  sulphur  by  the 
last  process,  lose  their  sulphur  slowly  on  the  application  of  steam, 
sulphureted  hydrogen  and  sulphurous  acid  being  formed.  Experi- 
ments made  by  Regnault  showed  that  desulphurization  is  effected 
more  perfectly  if  air  is  admitted.  Roasting  in  reverberatory  furnaces 
is  always  effected  by  the  oxygen  of  the  air  and  by  steam,  as  there  is 
no  fuel  used  which  contains  less  than  25  to  30  per  cent,  of  water. 
Superheated  steam  has  been  tried  in  different  ways  on  sulphurets 
with  the  highest  expectations,  but  with  no  better  results  for  practical 
use  than  are  given  in  the  ordinary  way  by  the  steam  obtained  from 
fuel.  It  may  be  useful  in  many  instances  to  have  more  steam  than  is 
thus  obtained,  but  this  increases  considerably  the  expense  of  roasting; 
as,  for  instance,  in  Patera's  application  of  steam  in  roasting  silver 
ores,  tried  principally  with  the  intention  of  expelling  antimony, 
arsenic,  etc.  Another  application  of  superheated  steam,  with  exclu- 
sion of  air,  is  Hagan's  method,  which  may  prove  sucsessful  on 
pyritous  ores,  having  at  the  same  time  the  advantage  of  being  a  very 
cheap  method.  d.  By  heating  with  metals,  alkalies  or  alkaline 
earths,  for  which  the  sulphur  has  a  greater  affinity.  The  affinity  of 
sulphur  for  the  following  metals  decreases  in  the  order  in  which  they 
stand,  being  strongest  for  the  first  and  weakest  for  the  last :  Copper, 
iron,  tin,  zinc,  lead,  silver,  antimony,  arsenic.  Each  of  these  metals 
can  be  desulphurized  by  the  next  preceding,  though  with  difficulty; 


1 2  INTRODUCTION. 

but  more  easily  by  one  further  off.  Practical  use  of  this  property  is 
made  in  smelting  galena  with  the  addition  of  metallic  iron  or  iron 
ore.  Sulphide  of  silver  in  crucibles  is  decomposed  by  stirring  the 
liquid  with  red  hot  iron.  In  a  cold  way  silver  sulphurets  and  chlo- 
rides are  decomposed  by  iron  in  amalgamating  pans,  the  chlorides  by 
iron  and  quicksilver.  Quicksilver  is  obtained  from  cinnabar  by 
heating  the  latter  with  lime,  which  takes  up  the  sulphur,  etc. 
e.  Carbon  has  no  great  affinity  for  sulphur ;  the  use  of  charcoal  for 
desulphurization  of  ores  is  therefore  an  inferior  method.  So  is  also 
the  use  of  carbonic  acid. 

Result  of  Desulphurization. 

The  direct  extraction  of  metals  from  sulphurets,  either  by  smelting 
or  amalgamation,  is  not  practicable.  In  smelting,  the  sulphurets 
melt  very  readily,  but  only  a  small  part  of  the  metal  is  obtained, 
while  the  greater  part  runs  out  combined  with  the  sulphur  as  matte. 
For  this  reason  the  roasting  of  sulphureted  ore  for  the  purpose  of 
smelting  is  indispensable  unless  iron  is  added,  which  desulphurizes 
the  ore.  Such  roasting  or  burning  takes  often  many  weeks  or  months. 
The  direct  amalgamation  of  sulphurets  gives  a  very  poor  result, 
except  in  the  case  of  silver  glance.  By  means  of  the  chemical  action 
of  sulphate  of  copper  and  salt,  the  silver  and  gold  sulphates  are  de- 
composed; but  no  process  has  yet  been  publicly  demonstrated  as 
really  practical  for  the  treatment  of  all  kinds  of  raw  sulphurets. 
The  desulphurization  is  therefore  still  a  most  important  preparation 
for  the  extraction  of  metals.  The  general  effect  of  roasting  is  that 
the  metals  become  oxidized.  Only  gold  and  silver  are  transformed 
into  a  metallic  condition;  -and  of  the  silver,  moreover,  a  large  per- 
centage is  always  found  as  a  sulphate,  even  when  the  roasting  is  well 
performed.  Some  of  the  silver  combines  as  an  oxide  with  antimony 
and  silica,  if  present.  All  the  oxides  obtained  by  desulphurization 
must  be  again  deoxidized  in  order  to  get  them  in  a  metallic  state. 


INTRODUCTION  13 


Means  of  Reduction  or  Deoxidation. 

Heating  alone  will  reduce  the  oxides  of  the  precious  metals  only. 
Oxide  of  gold  does  not  occur  in  nature,  neither  is  it  obtained  in  any 
of  the  metallurgical  processes.  Oxide  of  silver  is  also  unimportant; 
it  is  formed,  to  a  small  extent,  in  cupellation  (and  taken  up  by  the 
litharge),  in  smelting  silver  ores  combined  with  silica,  and  in  roasting 
silver  ores  in  the  presence  of  antimony,  arsenic,  etc. 

The  most  powerful  agent  of  reduction  is  carbon  (charcoal,  coke, 
etc.,)  and  carbonic  oxide.  In  all  smelting  in  blast  furnaces,  the  car- 
bonic oxide  is  the  real  reducer.  The  burning  coal,  under  the  in- 
fluence of  the  compressed  air,  produces  carbonic  acid,  melting  at  the 
same  time  the  ore;  the  carbonic  acid,  passing  through  the  glowing 
coal  above  the  melting  region,  gives  up  a  part  of  its  oxygen  to  the 
coal,  and  is  reduced  thereby  to  carbonic  oxide,  which  in  turn  takes 
up  oxygen  again,  from  the  metal  oxides,  reducing  them  to  a  metallic 
state ;  a  contact  of  ore  oxides  with  carbon  is  therefore  not  necessary 
for  the  purpose  of  reduction.  All  metals  do  not  retain  their  oxygen 
with  equal  tenacity,  but  some  part  with  it  much  more  easily  than 
others.  For  instance,  lead,  copper,  bismuth,  antimony,  cobalt  and 
nickel,  require  for  their  reduction  a  darker  or  lighter  red  heat;  while 
iron,  zinc  and  tin  are  reduced  only  at  a  white  heat.  But  also  hydro- 
gen and  carbureted  hydrogen,  created  by  the  burning  fuel,  are  power- 
ful reducing  agents. 

Metal  oxides  in  solution  are  reduced  and  precipitated  in  a  metallic 
condition  by  other  metals.  On  this  principle  copper  is  precipitated 
by  metallic  iron,  which  goes  into  solution  in  place  of  the  copper;  sul- 
phate of  silver,  in  Ziervogel's  process,  is  precipitated  by  copper,  etc. 
Also,  by  aid  of  electro-galvanic  stream,  metals  are  reduced  to  a  metal- 
lic state  from  their  solutions. 


14  INTRODUCTION. 


Desulphurization  of  Silver  Ores  not  Efficient. 

Although  by  mere  desulphurization  the  silver  is  to  a  great  extent 
converted  into  a  metallic  state,  this  is  not  always  its  most  suitable  con- 
dition except  for  smelting.  The  larger  part  of  all  silver  extracted  in 
the  United  States  is  obtained  by  amalgamation,  smelting  being  con- 
fined to  localities  where  the  ore  contains  such  a  high  percentage 
of  lead  that  its  amalgamation  is  impossible.  It  would  seem  as  if 
metallic  silver  should  amalgamate  more  easily  than  if  combined  with 
another  substance.  This,  however,  is  not  the  case.  The  silver, 
after  roasting,  is  generally  coated  with  the  oxides  of  volatile  base 
metals,  which  prevent  its  ready  amalgamation.  Moreover,  a  direct 
contact  between  quicksilver  and  silver  is  a  necessary  condition  for 
their  amalgamation.  A  momentary  contact  in  a  muddy  pulp  is  not 
always  successful.  The  chloride  of  silver,  however,  goes  into  solution 
and  unites  easily  with  the  quicksilver.  Hence,  in  most  instances,  it 
is  necessary  to  adopt  a  chloridizing  roasting. 

What  a  Chloride  is,  and  how  Chlorination  is 
Effected. 

The  term  chloride  is  applied  to  all  compounds  of  chlorine  with  a 
metal  or  other  radical.  Chlorine  is  a  greenish-yellow  g*s,  an  element- 
ary substance,  of  2.45  specific  gravity,  and  of  a  peculiar  and  disa- 
greeable odor.  It  is  not  found  free  in  nature,  but  always  in  combina- 
tion, principally  with  sodium,  forming  common  salt.  Metallic  chlo- 
rides are  of  frequent  occurrence.  Chlorine  is,  for  instance,  combined 
with  silver  as  horn  silver,  with  copper  as  Atacamite,  with  lead  as 
Kerasine,  Mendipite,  etc.;  also  with  quicksilver  as  Calomel. 

To  chloridize  ore,  that  is,  to  convert  the  metals  into  chlorides,  it  is 
necessary  to  produce  chlorine,  and  to  bring  it  in  intimate  contact  with 


INTRODUCTION.  15 

the  ore  particles.  The  cheapest  material  evolving— chlorine  is  salt  (chlo- 
ride of  sodium),  and  the  only  practical  way  of  separating  the  chlorine 
from  sodium  is  by  substituting  for  it  another  substance  for  which  the 
sodium  has  a  stronger  affinity.  The  cheapest  ingredient  for  this  pur- 
pose is  sulphuric  acid.  The  sodium  being  oxidized  to  soda,  unites 
with  the  sulphuric  acid,  forming  sulphate  of  soda,  while  chlorine  is  set 
free. 

For  the  treatment  of  ores  there  are  two  principal  methods  of  chlo- 
ridizing.  One  is  roasting  the  ore  with  salt  in  a  furnace ;  the  other  is 
the  "cold  chlorination."  Roasting,  at  first,  when  in  the  presence  of 
salt,  has  an  oxidizing  effect,  as  there  is  then  no  sulphuric  acid  present 
to  decompose  the  salt,  and  the  heat  alone  would,  if  increased,  volatilize, 
and  only  imperfectly  form  sulphuric  acid.  The  sulphurets  in  the  ore, 
under  the  influence  of  heat,  lose  a  part  of  their  sulphur  as  sulphur- 
ous acid  gas  ;  the  other  part  of  the  sulphur  oxidizes  to  sulphuric  acid. 
As  soon  as  this  is  formed  it  attacks  the  salt,  and  the  chlorine,  being 
set  free,  then  acts  on  metals,  metal  oxides,  sulphurets,  arseniurets  and 
antimonial  combinations,  forming  partly  metal  chlorides  and  partly 
chlorides  of  sulphur,  arsenic  and  antimony. 

The  other  mode  of  chloridizing  consists  in  the  employment  of  cold 
chlorine  gas  with  roasted  ores,  principally  desulphurized  gold  ores. 
The  chlorine  must  be  produced  here  separately,  and  conducted  into 
the  cold  ore  by  leaden  or  india  rubber  pipes.  The  ingredients  are: 
Salt,  sulphuric  acid  and  peroxide  of  manganese.  Salt  is  first  attacked 
by  the  sulphuric  acid,  and  hydrochloric  acid  and  sulphate  of  soda  are 
formed.  The  hydrogen  of  the  hydrochloric  acid  then  combines  with 
the  oxygen  of  the  manganese,  and  the  chlorine  escapes.  A  part  of 
the  chlorine  unites  with  the  manganese,  but  is  decomposed  again  by 
sulphuric  acid,  so  that  all  chlorine  is  expelled  from  the  salt,  leaving 
sulphates  of  soda  and  manganese  in  the  gas  generator.  The  chlorina. 
tion  of  gold,  unlike  that  of  silver,  is  difficult  to  effect  in  a  furnace 


16  INTRODUCTION. 

(Pg42)  for  the  reason  that,  if  formed,  the  gold  chloride  is  reduced  back 
to  the  metallic  state  at  a  low,  almost  dark  red  heat.  The  difference  be- 
tween hot  and  cold  chlorination  is  principally  found  in  the  fact  that, 
while  in  the  first  way  a  great  many  base  metal  chlorides  are  formed, 
the  cold  chlorine  combines  principally  with  the  free  metal,  with  silver 
and  gold;  while  the  other  metals,  being  oxidized,  are  not  decomposed 
by  the  chlorine.  Silver  oxide,  if  present,  is  decomposed  and  chlo- 
ridized. 

Chlorination  is  also  effected  by  chemical  decomposition  in  the  wet 
way,  as  practiced  in  the  Mexican  patio  amalgamation,  by  mixing 
with  the  ore  sulphate  of  copper  and  salt. 

Means  of  Separating  the  Metal  from  Chlorine. 

The  chloride  of  silver  can  be  melted  without  being  altered ;  chlo- 
rides of  gold  and  of  platinum  lose  all  their  chlorine  on  being  heated. 
In  this  way,  the  gold,  not  uniting  with  the  chlorine  in  the  heat,  can 
be  perfectly  refined  and  separated  from  other  metals  in  a  very  sim- 
ple way,  by  introducing  chlorine  gas  through  a  porcelain  or  earthen 
pipe  into  the  liquid  gold,  while  in  the  crucible.  The  silver,  copper 
and  other  base  metals  unite  with  the  chlorine  and  rise  to  the  surface 
as  chlorides.  Chloride  of  iron  exposed  to  air  and  heat,  as  is  the  case 
in  a  chloridizing  roasting,  loses  its  chlorine  and  is  changed  to  iron 
oxide.  The  chloride  of  copper  gives  up  only  a  part  of  its  chlorine. 
Heating  alone  has  therefore  no  practical  value  for  the  disengagement 
of  chlorine. 

The  most  effective  way  of  separating  the  chlorine  from  the  metal 
is  the  application  of  another  metal  for  which  the  chlorine  has  more 
affinity.  On  this  property  of  chlorine  is  based  the  amalgamation  of 
silver  ores  after  a  chloridizing  roasting  in  pans,  tubs  and  barrels,  and 
the  patio  amalgamation.  The  chloride  of  silver  in  the  ore  is  decom- 
posed, and  the  silver  set  free  during  amalgamation  in  iron  pans  by 


INTRODUCTION.  17 

the  metallic  iron  of  the  pan,  or  if  quicksilver  is  charged  at  the  same 
time  with  the  ore,  by  both  the  quicksilver  and  the  iron.  In  the  bar- 
rel amalgamation  the  silver  is  disengaged  by  metallic  iron,  and  in  the 
patio  amalgamation  by  quicksilver.  In  all  these  instances  the  silver 
being  deprived  of  its  chlorine,  alloys  with  the  quicksilver  and  forms 
the  amalgam. 

On  the  same  principal  the  metal  is  extracted  from  soluble  chlorides. 
The  proto-chlorides  are  all  more  or  less  soluble  in  water,  except  that 
of  silver,  which  is  quite  insoluble.  The  chloride  of  copper,  in  solu- 
tion, is  brought  together  with  metallic  iron,  or  conveyed  over  it.  The 
chlorine  of  the  copper  unites  with  the  iron,  and  the  copper  falls  in  a 
metallic  state,  ready  to  be  melted  into  bars,  after  being  washed, 
pressed  and  dried.  Indirectly,  the  silver  is  extracted  from  its  chlori- 
dized  state  by  dissolving  the  chloride  in  the  hyposulphites  of  soda,  of 
potash,  or  of  lime.  In  these  salts  the  silver  chloride  dissolves  very 
readily,  giving  a  clear  solution  of  a  very  sweet  taste,  out  of  which 
the  silver  is  precipitated  by  the  corresponding  alkaline  sulphides  as 
sulphide  of  silver. 

The  chloride  of  goldr  obtained  from  the  chlorination  of  gold-bearing 
sulphurets,  is  precipitated  by  sulphate  of  iron  in  such  a  way  that 
metallic  gold  results,  while  the  chlorine  combines  with  a  part  of  the 
iron. 

The  silver  is  easily  obtained  from  the  chloride  by  melting  it  with 
alkalies  ;  for  instance,  with  soda,  potash  or  lime.  The  chlorine  unites 
with  sodium,  calcium,  etc.,  and  the  silver  separates  on  the  bottom  of 
the  crucible.  If  there  is  not  a  sufficient  amount  of  the  alkalies  pres- 
ent, some  silver  will  be  lost.  In  most  instances  it  is  preferable  to 
mix  the  artificial  chloride  with  water  and  some  sulphuric  acid  and 
granulated  zinc,  or  zinc  sheet  if  smaller  quantities  are  being  operated 
on.  The  chloride  of  silver  by  degrees  changes  its  white  color  to  a 
dark  gray,  being  converted  into  the  metallic  state  in  a  short  time.  It 
is  reduced  to  metal  by  the  nascent  hydrogen.  After  the  sulphate  of 


18  BOASTING   OF    ORES. 

zinc,  which  is  formed  and  dissolved,  has  been  washed  away,  the  silver 
is  pressed,  dried,  and,  with  addition  of  some  soda  and  borax,  melted 
into  a  bar. 

In  the  same  way  as  from  a  sulphate,  silver  can  be  precipitated  by 
copper,  after  the  chloride  of  silver  has  been  dissolved  in  a  hot  solution 
of  salt,  as  is  done  in  Augustin's  process.  This  is  not  practicable  with 
the  argentiferous  solution  of  hyposulphite  of  soda. 

By  using  sodium  amalgam  and  iron  filings,  the  silver  chloride  is 
instantly  decomposed  and  silver  amalgam  formed. 

The  chloride  of  gold  is  precipitated  in  a  metallic  condition  by  the 
chloride  of  iron  (Fe  Cl),  the  consideration  of  which  is  important  in 
treating  sulphurets  by  chlorination. 


II.   ROASTING  OF  ORES. 

The  object  of  roasting  is  either  to  effect  chemical  changes,  as  re- 
quired for  amalgamation,  smelting,  etc.,  or  to  reduce  the  hardness  of 
the  ore,  in  order  to  make  it  easier  to  crush.  Boasting  for  the  latter 
purpose,  exposing  the  ore  to  the  fire  in  large  pieces,  is  more  properly 
termed  "burning."  The  beginning  of  smelting  is  under  all  circum- 
stances beyond  the  limits  of  roasting ;  therefore  all  roasting  furnaces 
in  which  the  regulation  of  heat  is  so  far  out  of  the  control  of  the 
roaster  that  a  partial  smelting  would  arise,  are  unfit  for  roasting. 
This  is  often  the  case  with  vertical  furnaces.  But  although  a  partial 
smelting  or  clotting  is  not  within  the  province  of  roasting,  and  in  all 
instances  is  very  injurious  to  the  result  of  subsequent  amalgamation 
or  precipitation,  it  is  nevertheless  applied  with  much  success  on  con- 


ROASTING   OF   ORES.  19 

centrated  ore  intended  for  smelting.  By  this  process  the  loose  sand 
assumes  a  compact  form,  the  gases  and  wind  penetrate  the  charge 
more  easily,  and  the  loss  in  metal  is  diminished. 

If  there  is  no  necessity  for  effecting  a  perfect  chemical  change  in 
the  ore,  or  if  roasting  is  required  for  smelting  purposes,  and  a  pow- 
dered form  is  not  admissable,  the  ore  is  taken  in  larger  or  smaller 
pieces — generally  not  below  the  size  of  a  hen's  egg — and  subjected 
to  roasting  either  in  open  heaps,  in  kilns,  or  in  vertical  or  reverbera- 
tory  furnaces.  In  roasting  in  heaps,  the  wood  is  first  placed  on  the 
ground,  sometimes  surrounded  by  a  wall  two  or  three  feet  high,  then 
the  ore  is  put  over  it.  Less  frequently  ore  and  wood  are  laid  in 
strata.  If  there  is  sufficient  sulphur  in  the  ore,  the  burning  will 
continue  without  addition  of  fuel  for  many  days  or  weeks.  It  is  evi- 
dent that  the  result  of  such  roasting  is  very  unequal,  the  outside 
being  more  oxidized  than  the  idside,  the  heat  greater  near  the  fuel 
than  further  off,  etc.  For  this  reason  such  ore  is  often  roasted  over 
several  times. 

In  vertical  furnaces  the  ore  is  laid  in  strata  alternating  with  fuel, 
or  there  are  several  fire-places  outside  the  furnace  so  arranged  that 
the  flame  is  conducted  by  the  draft  into  the  furnace.  A  modification 
in  construction  and  principle  is  the  Hagan  roasting  furnace,  in  which 
the  decomposition  of  superheated  steam  is  a  source  of  creating  heat 
and  a  decomposing  agent  at  the  same  time.  The  roasting  is  performed 
in  a  short  time,  and  with  proper  ore  and  pieces  of  the  right  size  the 
result  is  very  satisfactory.  It  is  also  a  cheap  process,  and  is  applied 
for  roasting  gold  quartz  holding  sulphurets,  the  amalgamation  of 
which,  without  roasting,  is  defective.  This  kind  of  roasting  would  be 
also  applicable  as  a  preparatory  for  amalgamating  silver  ores. 

In  most  instances  with  silver  ores  a  most  perfect  chemical  change 
is  a  condition  on  which  the  result  of  extracting  the  silver  depends, 
and  for  this  purpose  the  ore  must  be  pulverized,  in  order  to  effect  a 


20  BOASTING   OF    ORES. 

perfect  contact  between  ore  particles,  gases,  and  other  substances 
which  are  mixed  with  the  ore  for  certain  purposes. 

The  finer  the  ore  is  pulverized  the  more  effectual  will  be  the  in- 
tended purpose  of  roasting ;  but  there  are  so  many  disadvantages 
connected  with  finely  pulverized  ore,  that  it  is  preferable  to  pulverize 
the  ore  as  coarse  as  possible  without  interfering  with  a  good  final  re- 
sult. In  the  first  place,  the  waste  of  ore  in  crushing  increases  with 
the  fineness  on  account  of  dusting ;  the  expense  increases  also,  be- 
cause less  ore  will  be  pulverized,  and  a  still  greater  waste  will  occur 
during  the  roasting.  It  requires  more  extensive  dust  chambers  than 
coarse  crushing,  but  considering  the  very  variable  nature  of  ores,  and 
also  the  mode  of  extraction,  the  fineness  must  also  vary.  Generally 
in  pulverizing  roasting  ore  for  pan  amalgamation,  No.  40  wire  sieve 
(40  holes  to  the  running  inch)  is  used ;  also,  No.  35.  Working  the 
ore  raw  in  pans  without  grinding,  using  the  vitriol  and  salt,  No.  60 
has  been  applied  with  good  success,  but  the  crushing  was  performed 
dry,  in  order  to  avoid  the  loss  of  pulp  carried  off  by  water,  although 
a  sufficient  number  of  settling  tanks  would  retain  almost  all  of  the 
sloam ;  but  the  handling  of  such  fine  mud  is  very  inconvenient,  and 
by  no  means  a  clean  job.  Ore  intended  for  lixiviation  can  be  pulver- 
ized coarser  than  for  pan  amalgamation.  Sieve  No.  30  will  probably 
answer  in  most  cases ;  even  No.  25  is  used  successfully  (in  Mexico). 

The  roasting  of  pulverized  ore  is  now  performed  principally  in  con- 
tinuous self-discharging  furnaces,  or  in  mechanical  furnaces  by  charges, 
whereby  stirring  by  hands  is  avoided.  The  old  reverberatory 
have  fallen  into  disuse,  except  in  remote  localities,  or  they  are  em- 
ployed for  roasting  concentrated  gold  sulphurets.  The  continuous 
roasting  furnaces  are  of  diverse  construction ;  but  all  agree  in  one 
important  point,  that  the  pulverized  ore  is  conveyed  by  mechanical 
means  from  the  pulverizing  apparatus  direct  into  the  furnace,  and 
discharge  therefrom  in  the  same  proportion  as  fed  by  the  batteries  or 
other  pulverizing  contrivances, 


ROASTING   OF    ORES.  21 

Other  mechanical  furnaces,  not  continuous,  are  revolving  cylinders 
of  different  descriptions,  all  so  arranged  as  to  be  charged  at  once  with 
from  three  to  seven  tons,  and  the  whole  discharged  after  six  or  eight 
hours  roasting.  The  third  class  is  represented  by  the  old  reverberatory 
furnaces,  which  in  almost  all  cases  give  a  satisfactory  result,  if  properly 
treated.  All  have  a  horizontal  hearth  on  which  the  ore  is  spread;  all 
have  at  one  end  the  fire-place,  and  the  flue  at  the  other,  connected 
with  the  chimney.  Between  the  flue  and  fire-place  there  are  often 
two  or  three  hearths,  or  one  above  the  other.  In  accordance  with 
the  intended  mode  of  extraction,  the  ore  is  either  roasted  with  an 
addition  of  charcoal  powder,  whereby  the  silver  is  reduced  to  a 
metallic  state — a  procedure  of  no  practical  use — or  the  ore  is  sub- 
jected to  an  oxidizing  roasting,  with  the  principal  object  of  driving 
out  arsenic,  antimony  or  sulphur,  converting  at  the  same  time  the 
silver  into  a  sulphate  (Ziervogel's  process);  or  a  chloridizing  roasting 
is  effected,  that  is,  roasting  with  salt. 

The  roasting  of  gold  sulphurets,  and  especially  of  silver  ores,  is  of 
great  importance,  because  ores  of  this  class  which  require  roasting^ 
predominate  considerably.  This  part  of  metallurgy  deserves  special 
attention,  so  much  more  as  it  represents  the  foundation  of  subsequent 
manipulations  in  extracting  precious  metals,  either  by  amalgamation 
or  lixivation.  The  oxidizing  roasting  is  applicable  only  on  gold-bear- 
ing iron  sulphurets  of  the  best  quality,  and  never  on  silver  ores,  ex- 
cept sometimes  in  the  beginning  of  roasting,  then  for  the  Ziervogel's 
process  and  for  smelting  purposes. 

A   Chloridizing  Roasting  in    Reverberatory 
Furnaces. 

In  order  to  chloridize  the  silver,  an  addition  of  common  salt  is  in- 
dispensable. The  salt  furnishes  chlorine  for  that  purpose,  and  is  de- 
composed by  sulphuric  acid.  The  sulphuric  acid  is  created  by  the 


22  ROASTING   OP   ORES. 

decomposition  of  sulphurets  present  in  the  ore.  It  follows  that  if 
silver  ore  is  to  be  roasted  successfully  with  salt,  there  must  be  a  cer- 
tain percentage  of  sulphurets  in  it ;  otherwise  no  sulphuric  acid  can 
be  obtained,  and  consequently  no  chlorination,  or  at  least  only  an  im- 
perfect one,  can  be  effected. 

The  time  when  the  salt  should  be  introduced,  whether  at  the  same 
time  with  the  ore,  or  after  the  greatest  part  of  the  sulphur  and  arsenic 
has  been  expelled,  is  a  question.  Many  manipulators  consider  it 
essential,  especially  with  base  ore,  if  rich  in  antimony  and  arsenic,  to 
commence  the  roasting  without  salt  and  to  introduce  it  at  the  end  of 
the  operation,  one  hour  or  half  an  hour  before  discharge.  The  idea 
is  that  arsenic  and  antimony,  as  chlorides,  volatilize  and  influence 
the  silver  also  to  become  volatile,  causing  then  a  considerable  loss. 
Although  this  is  true,  it  seems,  however,  that  too  much  importance  is 
attributed  to  this  behavior  of  the  two  former  metals,  because  the  an- 
timony commences  to  escape  at  a  comparatively  low  temperature,  at 
which  little  of  the  salt  would  decompose;  and  on  the  other  hand,  if  no 
salt  is  present,  the  silver  volatilizes  with  the  antimony  anyhow;  this 
can  be  observed  if  the  ore  is  rich  in  both  metals  as  a  pink  colored 
coating  on  the  handles  of  the  hoes.  An  objection  to  the  intro- 
duction of  salt  at  the  end  of  an  operation  is  the  difficulty  of  a  per- 
fect mixing.  In  many  works  in  Mexico,  for  instance,  the  salt  is  used 
in  a  very  coarse  state,  and  then  not  only  that  during  the  last  hour  of 
chlorination  the  mixing  is  imperfect,  but  the  time  is  too  short  to  de- 
compose or  volatilize  the  coarse  particles,  and  after  the  charge  is 
drawn  out  a  great  deal  of  the  salt  remains  undecomposed.  Another 
objection  to  the  charging  of  salt  at  the  end  of  roasting  is,  that  very 
often  little  lumps  of  ore  are  formed  in  the  beginning  of  roasting  and 
remain  so  during  the  whole  operation,  sometimes  increasing  in  size. 
If  then  the  salt  is  added,  this,  of  course,  cannot  enter  the  inside  of 
the  lumps  and  the  chlorination  cannot  be  perfect ;  whereas,  if  the 
salt  had  been  introduced  at  the  commencement,  it  would  have  been 


ROASTING   OF   ORES.  23 

diffused  also  in  the  lumps  and  the  silver  therein  chloridized.  But  in 
case  there  is  so  much  lead  in  the  ore  that  it  commences  to  bake  at  an 
increased  heat,  then  it  is  advisable  to  roast  without  salt  and  add  it  in 
the  last  hour  of  the  roasting. 

The  most  perfect  mixing  of  salt  and  ore  is  doubtless  effected  in  a 
battery.  The  objection  that  choking  of  the  sieves  occurs  if  salt  is 
used,  seems  to  be  groundless,  unless  charged  in  a  moist  condition. 
Almost  all  mills  are  compelled  to  dry  the  ore  on  a  platform  near  the 
battery,  or  otherwise.  When  the  ore  is  spread,  the  necessary  per- 
centage of  salt  in  a  coarse  state  is  scattered  equally  over  the  ore, 
dried  and  crushed  together.  Some  use  half  of  the  salt  in  the  battery 
and  add  the  other  half  to  the  ore  in  the  furnace  before  discharging, 
but  the  benefit  of  this  procedure  is  not  evident.  It  is,  however,  dif- 
ferent with  the  mechanical  furnaces,  where  the  ore  is  conveyed  by 
screws  for  some  distance  to  the  furnace.  ;If  the  salt  is  continuously 
added  by  a  mechanical  arrangement  to  the  continuous  feed  of  ore  in 
the  required  proportion,  when  the  ore  enters  the  screw,  the  mixing  of 
salt  and  ore  as  it  moves  forward  under  the  screws  is  quite  sufficient, 
but  in  this  case  the  salt  has  to  be  dried  and  pulverized  separately  from 
the  ore.  A  very  good  arrangement  for  drying  salt  and  ore  is  the  re- 
volving cylinder.  Before  introducing  the  ore  into  the  furnace  the 
latter  must  be  gradually  heated  up,  which  may  take  ten  to  fifteen 
hours.  When  nearly  red  hot,  a  charge  of  dry  ore,  mixed  with  salt,  is 
brought  on  the  hearth  through  the  roof  and  spread  out  equally  by 
means  of  a  hoe.  The  fire  is  kept  up  moderately,  but  sufficient  flame 
must  be  seen  over  the  ore.  The  draft  is  lessened  by  the  damper,  and 
the  ore  stirred  diligently,  but  not  continually.  The  intervals,  how- 
ever, must  be  short. 

In  case  the  ore  contains  so  much  lead  that  it  commences  to  bake 
at  a  little  increased  heat,  this  must  be  kept  dark  red,  and  a  more  fre- 
quent stirring  is  required  for  the  first  two  or  three  hours.  By  de- 


24  ROASTING   OF   OEES. 

grees  the  ore  becomes  red  hot  and  the  burning  of  the  sulphur  in 
presence  of  a  great  amount  of  sulphurets  is  quite  lively.  One  part 
of  the  sulphur,  by  the  action  of  oxygen,  is  converted  into  sulphuric 
acid  and  combines  with  the  metals,  deprived  of  their  sulphur  or  ar- 
senic, to  sulphates.  The  period  of  the  formation  of  sulphates  is  very 
important  and  requires  some  time  before  it  is  finished.  If  there  is  a 
large  amount  of  sulphurets  in  the  ore,  the  burning  of  the  sulphur 
creates  so  much  heat  that  the  feeding  of  the  fire  must  be  stopped  al- 
most entirely  for  an  hour  or  two,  but  must  be  resumed  again  as  soon 
as  it  is  perceived  that  the  ore  commences  to  cool.  The  workman  stirs 
the  ore,  with  a  hoe  or  an  iron  rake,  back  and  forward  across  the 
hearth,  moving  it  from  the  bridge  toward  the  flue  and  back.  The 
formation  of  sulphates  still  continues  with  disengagement  of  sul- 
phurous gas.  The  ore  at  the  bridge  is  more  exposed  to  heat  than 
that  on  the  opposite  side,  and  the  roaster  is  obliged  to  change  the  ore 
by  raking  it  together  into  a  long  heap  extending  from  the  bridge  to- 
ward the  flue — not  in  the  middle,  but  nearer  the  working  door.  By 
means  of  a  shovel,  six  inches  by  twelve,  on  a  long  (12-foot)  iron 
handle,  the  roaster  takes  the  ore  from  near  the  bridge  and  .transfers  it 
toward  the  flue,  putting  it  behind  the  ridge  of  ore  until  he  reaches 
the  middle  of  the  furnace.  He  then  takes  the  other  end  of  the 
ridge  and  moves  it  toward  the  bridge.  After  this  the  stirring  is 
continued  in  the  usual  way.  As  often  as  the  roaster  stops  stirring, 
he  should  draw  furrows  across  the  hearth  from  twelve  to  fifteen 
inches  apart,  before  he  closes  the  door.  The  surface  of  the  ore  as- 
sumes then  a  wave-like  shape,  by  which  the  area  of  the  surface  is 
more  than  doubled  .and  the  heat  and  oxygen  have  so  much  more  ex- 
posed ore  to  a#t  upon.  This  is  important  enough  to  be  strictly  ob- 
served. The  sulphurets  (combination  of  sulphur  and  metal),  being 
now  converted  into  sulphates  (sulphuric  acid  and  metal  oxide)  react 
now  on  the  salt,  decompose  it  at  an  increased  heat,  and  set  the 
chlorine  free ;  a  mutual  exchange  takes  place. 


ROASTING    OF    ORES.  25 

Sulphate    of    Lead. 

Sulphate  of  lead  changes  partly  into  chloride  of  lead,  and  partly  it 
remains  as  a  sulphate.  This  fact  is  of  great  importance  for  the  sub- 
sequent pan  amalgamation.  All  chloride  of  lead  will  be  amalgamated 
and  depreciates  the  bullion,  while  the  sulphate  does  not  amalgamate. 
According  to  an  analysis  of  Mr.  Stetefeldt's  of  roasted  ore  (in  the 
Stetefeldt  furnace)  from  Ontario,  Utah,  all  lead  that  was  in  the  ore 
proved  to  be  in  the  state  of  a  sulphate.  This  occurs  also  in  rever- 
beratory  furnaces.  For  the  lixiviation  process  the  result  is  immaterial, 
because  'the  sulphate,  as  well  as  the  chloride  of  lead,  will  be  dissolved 
in  the  solving  solution  and  carried  out  with  the  silver  together.  At 
Plomosas,  Mexico,  the  ore  is  roasted  in  reverberatory  furnaces,  and 
almost  all  the  lead  remains  as  a  sulphate  after  roasting.  The  bullion 
from  the  lixiviation  contained  about  500  parts  of  lead  in  1,000,  while 
that  from  pan  amalgamation  is  free  of  lead. 

It  seems  that  a  high  roof,  27  or  30  inches  above  the  roasting  floor, 
is  more  favorable  for  the  formation  of  sulphate  of  lead  than  20 
inches  or  less.  In  the  former  case,  there  is  more  of  the  radiating  heat 
that  acts  on  the  ore ;  in  the  last,  the  ore  comes  too  much  in  contact 
with  the  flame.  The  chloride  of  lead  volatilizes  partly,  and  coming 
in  contact  with  air,  loses  one  part  of  its  chlorine  and  is  reduced  to  a 
combination  of  oxy-chloride  of  lead.  Sulphate  of  iron  and  sulphate 
of  copper  change  also  into  chlorides.  The  copper  chloride  becomes 
volatile,  colors  the  flame  blue,  emits  chlorine  gas  and  forms  subchlo- 
ride  of  copper.  The  chlorine,  set  free,  decomposes  the  sulphurets 
and  sulphates  of  silver,  and  creates  chloride  of  silver.  If,  during 
the  operation,  lumps  are  formed,  in  case  the  ore  was  not  dry  enough 
or  too  much  heat  was  applied  in  the  beginning,  they  must  be  crushed 
to  powder  by  a  hammer-like  iron  instrument  with  a  long  handle. 
As  soon  as  the  chlorination  begins,  after  three  or  four  hours,  a 
different  smell,  that  of  chlorine,  will  be  observed.  White  fumes 


26  ROASTING   OF    ORES. 


arise,  and  gases  and  vapors'arejevolved,  consisting  of  sulphurous  acid, 

chlorine,  hydrochloric  acid  gas,  chloride  of  sulphur,   of  iron  and  of 

copper. 

The  ore  increases  now  in  volume,  and  assumes  a  wooly 
condition.  Another  hour's  roasting  will  now  finish  the  chlori- 
nation.  This  last  hour's  stirring  requires  a  light  red  heat,  in 
order  to  destroy  as  much  as  possible  of  the  base  metal  chlo- 
rides. If  there  is  a  great  percentage  of  copper  and  other  base 
metals  in  the  ore,  the  roasting  may  require  more  time,  in 
order  to  decompose  the  chlorides  and  sulphates,  the  presence 
of  which  consumes  too  much  iron,  and  during  amalgamation 
in  barrels,  increases  the  heat  to  such  a  degree  as  to  cause  an  in- 
jurious division  of  the  mercury  into  small  particles  and  scum. 
The  base  metal  chlorides  are  reduced  by  the  iron  and  also 
amalgamated. 

The  changing  of  the  cooler  portion  near  the  flue  with  the 
hotter  part  at  the  bridge  must  be  repeated  two  or  three 
times  during  the  roasting  process.  When  finished,  after  five 
or  six  hours,  the  ore  is  drawn  out  and  discharged  through  the 
discharge-hole  in  the  bottom.  White  fumes  and  gases  are 
still  arising ;  and  if  left  in  a  heap  for  several  hours,  the 
chlorinatioii  is  still  going  on,  and  may  gain  5  to  6  per  cent,  of 
chlorination. 

1^  The  hoe  is  made    of    J-inch  wrought  iron,  6  inches  high 
and  eight  inches  wide.       The  rod  or  handle  must  be  fifteen 
feet  long  at  least.    This  would  render  the  instrument  heavy 
Fig.  i.   and  tiresome  to  handle ;  it  is  therefore  preferable  to  use  a 

piece  of  gas-pipe,  welding  it  together  with  the  rod  represented  in  Fig.  1. 

Necessary  Amount  of  Sulphurets. 

In  times  when  the  barrel  amalgamation  was  yet  practiced  in  Frei- 
berg (Saxony),  long  experience  showed  that  a  large  amount  of  iron 


ROASTING    OP    OEES.  27 

sulphurets  was  necessary,  in  order  to  decompose  the  amount  of 
salt  'required  for  the  chlorination.  One  hundred  parts  of  the  ore 
were  mixed  with  150  parts  of  borax  glass,  100  parts  of  common 
glass,  and  one  part  of  resin.  This  mixture,  melted  in  an  essay  cruci- 
ble, gave  a  button  of  matte  (sulphide  of  iron),  the  weight  of  which 
was  from  25  to  30  per  cent,  of  the  original  weight  of  the  ore.  If  less 
matte  was  obtained,  the  ore  was  considered  too  poor  in  sulphurets,  and 
more  pyrites  had  to  be  added. 

There  is  not  much  silver  ore  found  in  the  State  of  Nevada,  which 
would  give  25  per  cent,  of  matte  on  the  average;  and  as  there  are  no 
pyrites  to  be  obtained  for  this  purpose,  the  ore  must  be  roasted  as  it 
is.  When  starting  the  first  amalgamation  works  in  Nevada,  I  found 
from  six  to  eight  per  cent,  of  sulphurets  (different  kinds)  in  the  Corn- 
stock  ore,  which,  after  roasting,  contained  88  per  cent,  of  its  silver 
converted  into  a  chloride.  The  ore  from  the  Rising  Star  Mine 
(Idaho)  had  not  over  8  or  10  per  cent,  of  sulphurets,  still  there  was 
91  per  cent,  of  chloride  of  silver  found  after  roasting.  It  is,  how- 
ever, very  probable  that  from  silver  ores  containing  a  great  deal  of 
calc-spar,  or  heavy  spar,  a  less  satisfactory  result  might  be  obtained 
by  chloridizing  roasting,  if  no  more  than  6  per  cent,  of  sulphurets 
should  occur  in  them.  Some  copriferous  ores,  especially  if  other 
base  metals  are  present,  and  no  sulphur  (or  very  little),  will  give 
sometimes  a  good  chloridizing  roasting,  without  any  addition  of  green 
vitriol  or  other  sulphur  combinations. 

In  treating  ores  entirely  free  from,  or  with  a  very  small  percentage 
of  sulphurets,  the  want  of  sulphuric  acid  must  be  remedied  by  add- 
ing another  substance.  A  cheap  material  of  this  kind  is  found  in 
the  green  vitriol  or  copperas  (sulphate  of  iron),  of  which  1 J  to  3  per 
cent,  is  added  when  8  to  10  per  cent,  of  salt  is  used.  The  copperas 
is  first  calcined,  in  order  to  drive  out  its  water  of  crystalization,  by 
a  gentle  heat,  and  from  the  calcined  article,  not  the  crystalized,  is 


28  ROASTING   OF    ORES. 

taken  the 'above  percentage.  This  sulphate  acts  then  on  the  salt 'the 
same  as  if  it  were  created  in  roasting.  The  copperas  is  also  added  to 
arsenical  ores  free  from  sulphurets.  But  the  percentage  of  green  vit- 
riol to  be  added  depends  also  on  the  nature  of  the  gangue.  If  there 
is  a  great  deal  of  lime  in  the  ore  it  takes  up  sulphuric  acid,  forming 
sulphate  of  lime,  remaining  in  this  condition  through  the  process  of 
roasting  without  being  decomposed  further.  For  this  reason  calcerous 
ore  requires  as  much  more  geeen  vitriol  or  iron  pyrites  as  is  necessary 
to  transform  all  lime  into  a  sulphate.  Silica  or  quartz,  if  abundant, 
in  the  presence  of  steam,  decomposes  some  of  the  salt  when  red  hot, 
forming  silicate  of  soda  and  hydrochloric  acid,  the  importance  of 
which  is  shown  by  the  fact  that  gaseous  hydrochloric  acid,  in  contact 
with  metallic  silver,  unites  with  it  to  a  chloride.  It  behaves  in  a  like 
manner  with  sulphurets  and  arsenides,  of  which  the  most  are  decom- 
posed, forming  chlorides,  while  sulphur  and  arsesic  escape  combined 
with  hydrogen. 

Amount  of  Salt  to  be  used. 

Ores  containing  from  80  to  100  ounces  of  silver  per  ton  should  be 
mixed  with  10  per  cent,  of  salt.  This  is  about  the  quantity  consid- 
ered necessary  in  the  amalgamation  works  of  Europe.  Rich  ore  is 
often  roasted  with  20  per  cent,  of  salt.  If  all  the  chlorine  of  the  salt 
could  be  transferred  to  the  silver,  an  insignificant  amount  of  salt  only 
would  be  required  for  ores  containing  100  ounces  of  silver — not  more 
than  3J  pounds  to  the  ton;  but  in  consequence  of  the  different  ways 
in  which  the  chlorine  decomposes  and  unites  with  base  metals  and 
gases,  the  escape  of  chlorine  from  the  surface  of  the  ore  without  com- 
ing in  contact  with  the  silver,  etc.,  a  great  deal  more  of  the  salt  must 
be  applied. 

The  usual  amount  of  salt  used  for  ores  of  the  above  value,  is  from 
120  to  140  pounds  per  ton  of  ore ;  that  is,  from  6  to  7  per  cent.  It 


ROASTING   OP   ORES.  29 

is  not  advisable  to  take  less  than  6  per  cent.,  even  if  the  ore  be  poorer. 
There  are  instances,  however,  where  91  per  cent,  of  silver  has  been 
obtained  by  amalgamation  from  ores  which  were  roasted  with  only  5 
per  cent,  of  salt,  and  even  less.  There  was  no  natural  chloride  of 
silver  in  the  ore  when  treated  with  5  per  cent.  (Rising  Star  ore, 
Idaho.) 

As  the  salt  is  not  at  all  decomposed  before  the  formation  of  sul- 
phates commences,  or  only  to  a  very  small  extent,  it  is  also  in  this 
respect  immaterial  whether  the  salt  is  charged  at  once  with  the  ore, 
or  whether  it  is  introduced  two  hours  later,  unless  the  ore  is  of  such 
a  nature  as  would  bake  easily  on  a  little  increase  of  heat.  In  other 
cases,  however,  it  is  obvious  that,  taking  only  6  per  cent,  of  salt,  and 
employing  only  one  man  at  a  furnace,  a  perfect  mixing  in  a  short 
time,  as  ought  to  be  done  if  the  salt  is  charged  after  the  sulphur  is 
burned  off,  cannot  be  expected,  and  consequently  a  defective  result 
will  follow.  It  is  therefore  under  such  circumstances  important  to 
have  the  salt  and  ore  introduced  at  the  same  time. 

But  a  point  of  importance  is  the  time  when  the  salt  should  be 
added,  if  other  objects  are  in  view.  If  the  salt  is  added  to- 
gether with  the  ore,  or  after  the  sulphur  is  expelled  and  sulphates  are 
formed,  in  every  instance  the  base  metals  will  take  up  their  share 
of  the  chlorine,  and  therefore  more  salt  will  be  required.  But  as  the 
most  of  the  chlorides  are  volatile,  the  salt  is  the  means  of  getting  rid 
of  a  great  deal  of  the  metals  during  the  roasting,  which  in  some  in- 
stances is  not  very  desirable.  For  instance,  if  a  great  deal  of  anti- 
mony and  copper  is  in  the  ore,  more  or  less  chloride  of  silver  will  es- 
cape ;  sometime,  however,  only  a  small  percentage.  Treating  the  ore 
with  salt  from  the  beginning,  or  adding  it  two  hours  after  the  begin- 
ning, the  result  is  the  same. 

A  different  result  is  obtained  if  the  salt  is  added  after  all  the  base 
metals  are  desulphurized  and  oxidized.  Some  base  metals,  as  anti- 


30  ROASTING   OF   ORES. 

mony  and  arsenic,  will  be  volatilized  and  thus  gotten  rid  of,  but  not 
in  so  large  a  proportion  as  if  chloridized.  Iron  and  copper  remain 
entirely  in  the  ore,  while  both  are  volatile  as  chlorides.  The  roasting 
must  be  continued  at  a  light  red  heat  till  all  sulphates  are  decomposed 
and  the  metals  oxidized.  Applying  the  salt  after  the  dead-roasting, 
the  effect  differs  from  the  above  so  far,  that  the  base  metal  oxides 
now  are  not  chloridized,  or  only  to  a  small  extent,  while  the  silver 
alone  (some  of  which  appears  to  be  changed  to  a  metallic  state,  the 
most,  however,  remaining  as  a  sulphate)  will  be  chloridized.  But  in 
order  to  effect  this  chlorination,  from  1  to  2  per  cent,  of  green  vitriol 
must  be  added  in  order  to  accomplish  the  decomposition  of  all  the 
salt.  The  copper  is  lost  with  the  tailings  unless  smelted,  or  extracted 
by  diluted  sulphuric  acid. 

Condition  of  Metals  after  Roasting. 

The  silver  from  silver  sulphurets,  native  silver  and  other  combina- 
tions, is  turned  principally  into  a  chloride,  but  there  may  be  found  to 
a  small  extent  arsenate  and  antimonate  of  silver.  Copper  from  sul- 
phurets and  carbonates  becomes  an  oxide  and  a  small  part  of  it  is 
converted  into  chloride  and  sub-chloride,  but  the  latter  will  be  found 
in  much  larger  proportion  in  case  the  roaster  fails  to  increase  the  heat 
enough  to  decompose  the  chloride,  a  considerable  part  of  the  copper- 
chloride  votalizes  coloring  the  flame  blue.  The  lead  from  Galena  is 
turned  into  sulphate  of  lead,  and  in  presence  of  antimony,  partly  also 
into  antimonate  of  lead,  also  in  basic-chloride,  of  which  a  part  vola- 
tilizes likewise.  Antimony  volatilizes  during  the  roasting  to  a  great 
extent  as  antimonious  acid,  another  part  combines  with  other  metal 
oxides  to  antimonates.  Chloride  of  antimony  is  also  formed  and  is 
also  volatile.  Zinc,  from  zinc  blend,  is  reduced  to  oxide,  but  partly 
also  transformed  into  a  volatile  chloride.  Iron  and  arsenic  from 
pyrites  become  oxidized,  a  small  part  of  the  iron  remains  as  arsenate 
and  chloride. 


ROASTING   OF   ORES.  31 

Quartz  and  silicates   remain  unchanged.       Calc  and  heavy  spar, 
brown  and  fluor  spar  are  turned  into  sulphates. 


Permanent  Stirring  not   Essential. 

In  roasting  the  ore  with  salt,  a  continual  stirring  to  the  end  of 
the  process  is  not  a  necessary  condition  for  obtaining  a  good  result. 
This  depends  partly  on  the  time  and  partly  on  the  nature  of  the  ore. 
As  long  as  the  ore  is  not  uniformly  heated,  a  diligent  stirring  is  im- 
portant. The  ore  in  the  corners  is  too  often  neglected  while  the  sul- 
phur is  burning,  and  the  exposure  of  a  fresh  surface  to  the  oxygen 
of  the  air  requires  also  constant  work  \  but  as  soon  as  the  smell  of 
the  chlorine  is  perceptible,  the  stirring  can  be  carried  on  at  intervals 
of  from  eight  to  ten  minutes.  The  chlorine  which  is  evolved  in  the 
mass  has  better  opportunity  to  act  on  the  metals  than  if  constantly 
stirred,  whereby  more  chlorine  escapes  up  the  chimney  without  pro- 
ducing any  effect.  This  was  proved  by  a  comparison  of  the  work  of 
two  furnaces.  A  revolving  furnace  had  a  speed  great  enough  to  let 
the  ore  drop  constantly  through  the  flame  and  air,  while  the  com- 
mon furnace  was  managed  by  only  one  man,  and  stirred  at  intervals. 
Mr.  Atwood  found  15  per  cent,  less  chloride  of  silver  in  the  roasted 
ore  from  the  revolving  furnace.  The  blame  is  not  with  the  revolv- 
ing furnace,  but  with  the  speed.  It  proves,  however,  that  being  con- 
stantly exposed  to  the  air,  the  chlorine  escapes  with  less  effect  than 
in  the  common  furnace,  where  the  ore  is  allowed  to  rest  for  ten  or 
fifteen  minutes,  and  the  evolved  chlorine,  being  in  contact  with  the 
particles  while  passing  through  the  mass,  is  permitted  to  form  com- 
binations. O'Hara's  mechanical  furnace,  in  which  the  ore  is  com- 
paratively but  little  stirred,  gave  91  to  94  percent,  of  chloride  of  sil- 
ver. A  mixture  of  ore,  sawdust  and  salt,  formed  into  bricks  and 
calcined,  showed  the  silver  as  a  chloride  through  the  whole  mass, 


32  ROASTING   OF    ORES. 

where,  as  a  matter  of  course,  the  inside  did  not  come  into  direct  con- 
tact with  air.  Constant  shoveling  is  necessary  with  ore  of  such  a 
nature,  as  it  would  bake  if  not  stirred. 

Signs  of   a  good    Chloridizing    Roasting. 

A  good  chloridizing  roasting  should  give  over  90  per  cent,  of  the 
silver  converted  into  chloride  of  silver,  and  show  as  little  as  possible 
of  base  metal  chlorides.  To  ascertain  the  amount  of  chloride  of  sil- 
ver at  the  end  of  the  roasting,  it  is  necessary  to  make  two  assays. 
Several  pounds  of  ore  are  taken  from  different  parts  of  the  furnace? 
well  mixed  and  sifted.  From  this,  two  one-half  ounce  assays  are 
weighed  out,  and  one  (No.  1)  prepared  for  the  fire  assay  as  usual. 
The  other  half  ounce  (No.  2)  is  introduced  carefully  into  a  small  filter 
in  a  glass  funnel.  The  filtering  paper  must  project  about  one  inch 
above  the  ore.  A  solution  of  hyposulphite  of  soda  is  then  poured 
over  the  ore  in  the  filter,  and  this  is  continued  as  long  as  a  precipitate 
is  obtained  on  adding  one  or  two  drops  of  a  solution  of  sulphide  of 
sodium  to  the  filtered  liquid.  This  is  best  tried  in  a  glass  tube.  If, 
after  filtering  the  addition  of  sulphide  of  sodium  to  the  lixivium 
does  not  produce  a  precipitate,  or  only  a  very  slight  one,  so  that  the 
liquid  assumes  only  a  little  darker  color,  without  losing  its  transpa- 
rency, the  assay  is  then  leached  with  warm  water  and  the  filter  taken 
off,  placed  into  a  porcelain  or  sheetiron  dish,  dried,  and  the  filter 
paper  burned  above  the  sample,  or  with  the  sample  in  the  muffle  at  a 
low  heat.  The  sample,  with  the  ashes  of  the  filter,  is  fluxed  like  the 
other  half  ounce,  and  both  crucibles  placed  into  the  assay  furnace. 

The  operation  takes  less  time  if  the  hyposulphite  solution  is  used 
in  a  warm  state,  but  not  boiling.  All  chloride  of  silver,  and  also  sul- 
phate of  silver,  if  present,  is  dissolved  by  the  hyposulphite  and  carried 
off,  besides  the  base  metal  chlorides.  The  two  assays,  when  ready, 
are  compared,  and  the  difference  shows  the  silver  which  was  convert- 


ROASTING   OF   ORES.  33 

ed  into  a  chloride.  For  instance,  if  No.  1  assayed  83  ounces  per  ton 
and  No.  2  from  the  filter  4  ounces,  the  difference,  79,  is  that  part 
which  became  chloridized.  That  is, 

83:79  =  100:  x  =  95  per  cent. 

If  there  should  be  gold  in  the  ore,  this  must  be  subtracted  from 
both  assays,  because,  although  the  amount  of  gold  would  be  equal  in 
both  assays,  the  chlorination  result,  as  it  should  be,  must  come  out 
higher  after  the  gold  is  subtracted. 

The  filtering  operation,  if  properly  attended  to,  takes  one  hour's 
time  or  more,  and  if  a  great  deal  of  base  metals  is  present  it  takes  also 
more  hyposulphite  salt. 

Hyposulphite  of  soda  is  a  crystalized  salt,  of  which  five  or  eight 
ounces  may  be  dissolved  in  a  quart  of  water,  warm  or  cold.  This  is 
the  solving  solution.  It  dissolves  the  chloride  of  silver  much  quicker 
than  a  common  salt  solution  and  is  more  convenient  then  to  operate 
with  ammonia.  If  the  ore  sample  in  the  filter  is  treated  with  the 
hyposulphite  solution,  it  takes  in  the  beginning  a  great  deal  of  soluble 
chlorides  and  sulphates,  if  present,  but  after  a  while  always  less  and 
less ;  and  in  order  to  find  out  when  all  chlorides  are  extracted,  a  few 
drops  of  sulphide  of  sodium  or  of  calcium,  as  before  described,  are 
added,  and  if  no  precipitation  is  observed,  the  leaching  is  finished. 

For  sulphide  of  sodium  take  five  or  six  ounces  of  soda,  or  more,  if 
a  large  crucible  is  at  hand,  melt  it  at  a  good  heat,  and,  when  liquid, 
introduce  just  as  much  sulphur  (brimstone)  at  intervals  in  small 
pieces,  giving  time  for  the  boiling  up  to  subside.  Pour  out  on  an 
iron  plate  and  dissolve  in  water.  It  takes  several  hours  before  the 
solution  appears  perfectly  clear,  showing  a  yellow  color  above  the 
black  sediment.  When  drawn  off  it  is  ready  for  use.  Much  better 
than  sulphide  of  sodium  is  sulphide  of  calcium.  It  is  easily  made  by 
boiling  in  an  enameled  iron  kettle  3  parts  of  fresh  burned  lime  and  1  £ 


34  BOASTING   OF   ORES. 

parts  flower  of  sulphur  with  water,    enough  to  keep  it  thinly  boiling 
for  an  hour.     When  cold  the  clear  dark  red  liquid  can  be  drawn  off. 

Toward  the  end  of  the  roasting  yery  little,  if  any,  sulphate  of  silver 
will  be  found  in  the  ore;  but  if  a  small  percentage  of  it  should  remain, 
it  may,  for  the  purpose  of  amalgamation  or  extraction,  be  considered 
equal  to  the  chloride  of  silver ;  for,  as  soon  as  it  dissolves  in  water,  it 
becomes  a  chloride,  precipitated  by  the  salt,  of  which  a  part  is  always 
yet  found  undecomposed  in  the  ore.  To  obtain  a  general  idea  of  the 
amount  of  soluble  base  metals,  chlorides  and  sulphates,  it  is  sufficient 
to  put  a  small  sample  of  about  half  an  ounce  on  the  filter  as  before, 
and  to  leach  it  with  hot  water.  The  leach  obtained  is  tried  again 
with  the  sulphide  of  sodium.  A  thick  precipitate  shows  that  a  large 
amount  of  soluble  chlorides  is  in  the  roasted  ore.  If  a  reaction  of 
copper  is  expressly  desired,  ammonia  should  be  used  in  place  of  the 
sulphide  of  sodium.  In  presence  of  much  iron  the  precipitate  will 
appear  brown.  This  precipitate  must  smell  strongly  of  ammonia.  If 
copper  is  present,  a  clear  blue  liquid  will  be  seen  above  the  iron  pre- 
cipitate after  some  time;  or  the  whole  may  be  brought  on  a  filter  to 
separate  the  liquid  from  the  precipitate. 

To  Regulate  Chloridizing  Roasting. 

The  main  points  which  have  to  be  considered  at  a  chloridizing 
roasting  are :  The  time  of  roasting,  the  temperature,  the  amount  of 
salt  and  the  loss  of  silver,  all  with  reference  to  the  percentage  of  chlo- 
ride of  silver.  The  first  thing  necessary  to  know,  is  the  amount  of 
silver  contained  in  the  ore — for  this  purpose  a  reliable  average  sample 
must  be  taken,  including  the  salt  if  already  mixed  ;  this  sample  to  be 
No.  1.  It  must  be  taken  either  while  it  is  charged  in  the  furnace, 
or  at  the  battery  every  10  or  15  minutes  while  it  drops  from  the 
sieves.  The  roasting  should  be  carried  oil  at  a  moderate  heat,  as 
described  (page  21,) 


BOASTING   OF   ORES.  35 

After  three  or  four  hours,  when  the  ore  assumes  a  wooly  condition, 
other  samples  are  taken  from  different  parts  of  the  furnace,  mixed 
together,  and  the  assay  sample  therefrom  marked  No.  2.  One  hour 
later  a  third  sample  is  taken,  No.  3 ;  half  an  hour  after  this,  No.  4, 
and  before  the  discharge,  or  during  the  discharge,  No.  5.  The  tem- 
perature of  the  last  hour  should  be  a  moderate  light  red  heat.  In 
case  there  is  a  double  or  treble  hearth  furnace,  one  sample  more  should 
be  taken  from  the  hearth  next  to  the  finishing  hearth,  just  before 
moving  the  charge  over  to  the  finishing  hearth,  but  as  there  are  three 
charges  on  three  hearths,  the  samples  have  to  be  taken  of  one  charge 
as  it  progresses  from  one  hearth  to  the  other. 

While  the  next  charge  is  treated  in  the  same  way,  but  a  higher 
heat  applied,  the  samples  already  taken  must  be  assayed.  In  order 
to  find  the  progress  of  chlorination,  the  assay  of  No.  1  is  not  needed, 
it  serves  only  to  ascertain  the  loss  of  silver  during  the  roasting.  One 
assay  of  No.  1,  however,  would  not  be  sufficient,  because  the  weight 
of  the  ore  changes.  For  instance:  the  iron  sulphuret  (pyrites)  loses 
its  sulphur,  which  is  replaced  by  the  lighter  oxygen,  and  weighs  then 
33  per  cent,  less,  but  in  taking  up  chlorine,  in  place  of  the  oxygen, 
it  gets  much  heavier  than  it  was  when  a  sulphuret,  gaining  35  per 
cent.,  but  a  great  deal  of  the  chloride  of  iron  volatilizes.  For  this 
reason  two  assays  of  No.  1  are  weighed  out,  one  to  be  assayed  in  the 
usual  way,  to  find  out  the  amount  of  silver  per  ton,  and  the  other 
half  ounce  is  subjected  to  roasting  under  the  muffle,  in  a  roasting  cup 
for  one  hour  and  a  half  or  two,  at  about  the  same  heat  as  in  the  fur- 
nace, then  weighed  out  and  noted  how  many  grains  were  lost  or 
gained,  as  compared  with  the  original  240  grains. 

The  other  samples  are  assayed  to  ascertain  how  much  of  the  silver 
was  chloridized. 

If,  then,  all  the  assay  samples  showed  a  progress  in  chlorination,  so 
that  No.  5  was  the  highest,  but  still,  not  satisfactory,  it  indicates  that 


36  BOASTING   OF   ORES. 

either  the  time  was  too  short,  or  the  heat  too  low,  or  perhaps  not 
enough  salt;  these  assays  show  also  the  relative  loss  of  silver. 

While  these  assays  were  made,  the  next  charge,  as  before  men- 
tioned, was  to  be  conducted  at  a  somewhat  increased  heat,  and  sam- 
ples taken  as  before.  If  now,  for  instance,  the  last  two  samples 
should  show  the  same  percentage  of  chlorination,  it  were  evident 
that  the  roasting  was  carried  on  too  long.  In  case  the  chlorination 
were  still  too  low,  and  the  last  one  the  highest,  then  the  roasting 
should  be  tried  one  hour  longer — eventually  more  heat  or  more  salt, 
but  always  controlled  by  assays.  Such  investigation  may  take  two 
or  three  days,  or  more. 

To  ascertain  the  loss  of  silver,  an  average  sample  is  taken  when 
the  roasting  is  finished,  and  for  an  assay,  weigh  out,  not  half  an  ounce, 
but  so  many  grains  as  there  were  found  after  roasting  of  No.  1  under 
the  muffle,  and  this  weight  considered  as  half  an  ounce.  If  then,  for 
instance,  the  first  assay  of  No.  1  should  give  83  ounces  per  ton,  and 
that  of  the  roasted  ore  78,  that  is  five  ounces  less,  the  loss  of  silver 
by  roasting  would  be  2^=6  per  cent. 

The  loss  can  be  considerably  diminished  by  a  proper -arrangement 
of  sufficient  dust  chambers,  in  which  also  the  volatile  metals  could  be 
condensed. 

Means  of  Destroying  Base  Metal  Chlorides. 

It  is  very  difficult  to  get  rid  of  all  the  base  chlorides.  They  are 
formed  under  the  action  of  chlorine  and  hydrochloric  acid.  The  most  of 
the  metal  chlorides  are  volatile,  and  a  part  is  carried  off  through  the 
chimney.  Another  part  of  the  chlorides  gives  off  some  of  its  chlo- 
rine, whereby  sulphates,  undecomposed  sulphrets,  antimonates  and 
arsenates  are  chloridized.  Chlorides  which  are  disposed  to  transfer 
chlorine  to  other  metals  in  combination  with  sulphur  or  arsenic,  are: 
the  proto  chloride  of  iron  and  of  copper,  the  chlorides  of  zinc,  lead 
and  cobalt.  When  in  this  way  the  most  of  the  metals  are  cUoridized, 


ROASTING   OP   ORES,  37 

the  base  metals,  principally  iron  and  copper,  are  losing  their  chlorine 
gradually,  being  first  converted  into  sub-chlorides,  and  then  into 
oxides.  The  roasting  for  this  purpose  must  continue  with  increased 
heat,  even  when  the  chlorination  of  the  silver  is  finished.  At  an  in- 
creased heat,  the  base  metal  chlorides  lose  their  chlorine,  while  the 
chloride  of  silver  remains  undecomposed,  unless  a  very  high  tempera- 
ture should  be  applied.  This  process  requires  a  long  time,  conse" 
quently  also  more  fuel.  The  decomposition  of  these  chlorides  is 
greatly  assisted  by  the  use  of  5  to  6  per  cent,  of  carbonate  of  lime  in 
a  pulverized  condition.  Lime  does  not  attack  the  chloride  of  silver, 
but  it  is  not  advisable  to  take  too  much  of  it,  as  it  would  interfere  to 
some  degree  with  the  amalgamation.  The  pulverized  lime  rock  must 
be  charged  toward  the  end  of  the  roasting.  First,  two  per  cent,  is 
introduced  by  means  of  a  scoop,  the  whole  well  mixed,  and  then  ex- 
amined either  with  sulphide  of  sodium,  or  in  the  following  way: 

A  small  portion  of  the  roasted  ore  is  taken  in  a  porcelain  cup  or 
glass,  and  mixed  with  some  water  by  means  of  a  piece  of  iron  with 
a  clean  metallic  surface.  If  the  iron  appears  coated  red  with  copper, 
some  more  lime  must  be  added.  In  place  of  iron — especially  if  no 
copper,  but  some  other  base  metal  is  present — some  quicksilver  is 
mixed  with  the  sample.  In  the  presence  of  base  metal  chlorides,  the 
quicksilver  is  coated  immediately  with  a  black  skin. 

When  endeavoring  to  expel  the  base  metals  by  heat,  the  loss  of 
silver,  in  presence  of  much  antimony,  lead  and  copper,  should  be  in- 
vestigated very  carefully.  Under  certain  circumstances  it  is  not  un- 
common to  find  a  loss  of  even  50  per  cent,  of  the  silver,  if  the 
chloridizing  roasting  is  carried  on  at  a  high  heat  for  a  great  length  of 
time.  The  loss  increases  with  the  duration  of  roasting  and  with  the 
degree  of  temperature.  When  such  ore  is  under  treatment,  it  is  nec- 
essary to  take  samples  during  the  roasting,  and  to  examine  the  same 
3 


ROASTING   OF    ORES. 

for  the  amount  of  chloride  of  silver,  and  also  for  its  loss,  and  to  stop 
roasting  when  the  highest  percentage  of  chloride  of  silver  is  obtained 
without  reference  to  the  condition  of  base  metals. 

Steam  Decomposes  Base  Metal  Chlorides. 

The  formation  of  base  metal  chlorides  can  be  avoided  by  a  proper 
but  more  expensive  roasting.  It  requires,  first,  an  oxidizing  roast- 
ing, with  or  without  the  application  of  steam.  This  roasting  must 
continue  until  all  the  metals  are  desulphurized  and  converted  into 
oxides.  When  this  is  accomplished,  salt  and  green  vitriol  are  added, 
and  the  roasting  continued  until  all  the  silver  is  chloridized. 

There  is  also  a  very  good  way  of  getting  out  a  good  deal  of  the 
base  chlorides  of  the  ore  before  the  silver  is  amalgamated  or  ex- 
tracted, by  leaching  the  ore  with  hot  water. 

Application  of  Steam  in  Roasting. 

The  application  of  steam  in  roasting  is  advantageous,  for  the  rea- 
son that  hydrochloric  acid  is  created  by  the  decomposition  of  chlo- 
rides, which  in  turn  decomposes  the  sulphurets.  The  hydrogen  de- 
composes also  the  chloride  of  silver,  which,  upon  being  reduced  to 
metallic  condition  by  its  affinity  for  chlorine,  in  turn  decomposes  the 
hydrochloric  acid.  The  silver  may  thus  change  repeatedly  from  a 
metallic  condition  to  a  chloride,  while  the  base  metal  chlorides  are  re- 
duced to  oxides,  and  in  that  state  do  not  interfere  with  the  amalga- 
mation or  precipitation.  The  application  of  steam,  however,  requires 
a  great  deal  more  fuel  during  the  roasting.  Taking  the  moisture  of 
the  fuel  into  consideration,  there  is  no  roasting  done  without  steam, 
although  with  a  limited  quantity. 

Repeated  experiments  in  California,  Nevada  and  Mexico,  applying 
steam  through  the  bridge  of  the  furnace,  divided  into  four  or  five 
jets,  never  proved  to  be  of  such  an  advantage  as  to  be  continued,  or 
permanently  introduced  where  rebellious  ores  are  treated. 


ROASTING   OF   ORBS.  39 

Chloridizing    Roasting  of   Silver   Ore    Containing 

Lead. 

Lead  has  a  bad  influence  in  amalgamation,  lixiviation,  and  in  the 
roasting  itself,  causing  a  baking  of  the  ore  at  the  slightest  undue 
rising  of  the  temperature.  The  chloride  of  lead  amalgamates  easily, 
especially  in  iron  pans.  Ores  with  8  to  15  per  cent,  of  lead  still  allow 
of  a  successful  roasting.  A  part  of  the  formed  chloride  of  lead  escapes 
in  gaseous  form,  another  part  is  reduced  by  degrees  to  oxy-chloride  of 
lead.  This  latter  combination  goes  mostly  into  the  amalgam.  If 
there  is  .more  lead  in  the  ore  than  15  per  cent.,  it  gives  sometimes, 
according  to  its  nature,  as  much  as  85  per  cent,  of  silver,  and  the 
retorted  amalgam  is  submitted  to  cupellation  in  order  to  separate  the 
lead.  If,  as  already  stated  (page  25),  the  lead  is  transformed  into  a  sul- 
phate, it  does  not  amalgamate.  This  explains  the  reason  why  some 
bullion  of  plumbiferous  silver  ores  is  free  of  lead,  while  other  bullion 
again  contains  sometimes  600  parts  of  lead  in  a  thousand,  according 
to  the  condition  of  the  lead  in  the  roasted  ore.  As  yet  no  investiga- 
tion has  been  made  how  to  direct  the  roasting  in  order  to  convert  all 
lead  into  a  sulphate.  Carbonate  of  lead  as  well  as  galena,  form, 
under  certain  circumstances,  sulphate,  and  vary  little  of  chloride  of 
lead ;  sometimes  it  is  the  reverse. 

In  Hungary  (Offenbanya),  black  copper,  containing,  besides  the 
silver,  10  per  cent,  of  lead,  was  subjected  to  a  chloridizing  roasting. 
The  pulverized  copper  is  mixed  with  12  per  cent,  of  salt,  1  per  cent. 
of  green  vitriol,  and  3  per  cent,  of  saltpetre.  The  saltpetre  oxidizes 
the  lead  to  a  sulphate,  which  in  the  subsequent  barrel  amalgamation 
does  not  enter  the  amalgam.  Here  saltpetre  is  added  for  the  pur- 
pose of  oxidizing  the  lead  to  a  sulphate  in  presence  of  salt.  It  is 
possible  that  an  addition  of  50  or  60  Ibs.  of  pero^Jde  of  manganese 
a  ton  of  roasting  ore  may  have  a  great  influence  in  this  respect;  also 
the  construction  of  the  furnace  is  not  immaterial  (page  62.) 


40  ROASTING    OF    ORES. 

Difference  in  Roasting  Ore  for  Pan  Amalgamation 

as  Compared  with  that  for  other  Modes 

of  Extraction. 

The  roasting  of  silver  ores,  if  imperfect,  will  give  a  better  result  by 
amalgamating  in  an  iron  pan  than  in  wooden  barrels,  or  by  lixivia- 
tion.  This  is  due  to  the  better  decomposition  of  undecomposed  sul- 
phurets  under  the  grinding  muller.  The  roasting  for  pan  amalgama- 
tion is  therefore  less  delicate.  However,  when  once  at  work,  it  is 
always  better  to  do  the  roasting  properly;  but  it  is  not  necessary  to 
sift  the  ore  after  roasting  in  order  to  separate  the  lumps  from  the 
mass,  as  is  done  with  the  barrel  amalgamation,  except  to  prevent 
nails  from  coming  into  the  pan.  The  formation  of  such  lumps,  how- 
ever, must  be  avoided  as  much  as  possible.  Imperfect  roasting  in 
the  presence  of  base  metals  gives  in  pans  always  a  low  fineness  of 
bullion,  as  well  as  in  the  lixiviation  process. 

Roasting  Charges. 

The  charges  of  ore  in  reverberatory  furnaces  in  Europe  are  generally 
limited  to  five  or  six  hundred  pounds.  This  quantity  can  be  handled 
very  conveniently  and  all  the  theoretical  details  strictly  carried  out. 
The  result  of  chlorination  is  very  perfect ;  but,  for  all  this,  the  econ- 
omy here  in  the  United  States  is  different  with  the.  different  circum- 
stances. The  roasting  charges  here  are  generally  1000  to  2000 
pounds,  and  if  the  chlorination  comes  up  to  90  or  95  per  cent.,  the 
operation  can  be  considered  very  satisfactory. 

Roasting  of  Silver  Ores  Containing  Cold  and  Cop- 
per, to  be  Treated  after  Roasting  by  Chlorine 
Cas  and  Lixiviation. 

It  is  not  absolutely  necessary  for  this  process  to  have  the  ore 
roasted  with  salt,  but  it  has  been  found  that,  on  account  of  different 
earths,  an  addition  of  1  or  2  per  cent,  of  salt  produces  a  better  result. 
This  process  extracts  copper,  gold  and  silver,  each  of  which  is  obtained 


.      ROASTING    OF    ORES.  41 

separately;  but  it  makes  a  difference  in  roasting,  whether  the  copper 
is  intended  to  be  saved  or  not,  as  in  many  localities  the  copper  is  at 
present  of  no  value,  or  the  old  iron  for  precipitation  is  too  expensive. 

The  ore  is  crushed  dry  through  a  sieve  of  forty  holes  to  the  run- 
ning inch.  Some  ore  allows  also  thirty  holes  to  the  inch.  In  case 
the  ore  contains  so  much  clay  or  talc  that  no  leaching  is  admissible, 
the  ore  is  crushed  wet,  separated  from  slime,  and  dried.  Eight  hun- 
dred to  one  thousand  pounds  are  charged,  and,  according  to  the  quality 
of  the  ore,  the  heat  raised  quickly  or  slowly  to  a  bright  red  heat. 
This  is  an  oxidizing  roasting,  consequently  much  stirring  is  required. 
Ores  with  but  few  sulphurets  appear  sufficiently  well  roasted  after 
three  hours ;  other  ores,  containing  an  abundance  of  sulphurets,  take 
from  five  to  six  hours  and  more  before  all  sulphur,  arsenic  and  anti- 
mony are  expelled ;  but  the  arsenates  and  antimonates  formed  by  the 
two  last  are  not  volatile.  When  desulphurized,  1  or  2  per  cent,  of 
salt  is  thrown  in  the  furnace  and  mixed  with  the  ore  as  intimately  as 
possible.  Three-quarters  of  an  hour  after  the  addition  of  the  salt,  and 
with  only  a  moderate  heat,  the  roasting  is  finished.  Ores,  not  rich 
in  sulphurets,  may  be  mixed  with  salt  when  charged. 

If  it  is  intended  to  extract  the  copper  also,  this  must  be  transformed 
into  chloride  of  copper.  To  accomplish  this,  two  things  must  be  ob- 
served— first,  no  oxide  of  copper  should  be  formed  during  the  roast- 
ing; and  second,  more  salt  must  be  used.  Stechiometrically,  each 
pound  of  copper  requires  1.84  pounds  of  salt  to  form  a  chloride,  pro- 
vided all  chlorine  is  taken  up  by  the  copper ;  but  as  this  is  not  the 
case,  as  a  great  deal  of  chlorine  is  also  absorbed  by  other  metals,  etc., 
it  follows  that  at  least  two  pounds  of  salt,  if  not  more,  must  be  taken 
for  each  pound  of  copper  in  the  ore.  For  most  localities  such  a  quan- 
tity of  salt  could  not  be  used  on  account  of  the  difficulty  in  obtaining 
it.  It  may  be  mentioned  here  that  if  the  brine  remaining  after  the 
copper  has  been  precipitated  by  iron,  should  be  condensed  by  evapo- 


42  ROASTING  OF  ORES. 

ration,  exposing  it  to  the  heat  of  the  sun,  which  might  be  practicable 
on  the  Pacific  Coast  in  the  dry  season,  the  condensed  salt,  consisting 
of  chloride  of  iron,  could  be  added  to  the  ore  as  a  chloridizer,  whereby 
a  considerable  percentage  of  salt  would  be  saved. 

In  roasting  with  salt  with  reference  to  extracting  the  copper,  the 
ore  is  first  roasted  for  itself  at  a  low  temperature,  so  as  not  to  decom- 
pose the  sulphates  by  a  bright  red  heat,  but  long  enough  to  decompose 
all  sulphurets.  When  this  is  accomplished,  the  salt  is  introduced  and 
the  roasting  finished  in  one-half  to  three-quarters  of  an  hour  there- 
after. 

The  process  of  chloridizing  the  ore  with  chlorine  gas  after  an  oxi- 
dizing roasting,  for  the  extraction  of  gold  and  silver,  and  eventually 
the  copper  is  now  with  much  better  result  carried  out  by  O.  Hof- 
mann's  process,  (page  122);  but  the  roasting  must  be  a  chloridizing  one, 
with  the  required  amount  of  salt,  (page  28.) 

Chloridizing  Roasting  of  Silver  Ores  Containing 

Gold,  without  Subsequent  Treatment 

by  Chlorine  Gas. 

Generally  the  ore  containing  silver  and  gold  is  roasted  with  salt, 
converting  thereby  the  silver  into  a  chloride,  while  the  gold  remains 
in  a  metallic  condition.  This  mode  of  roasting  is  quite  satisfactory 
for  the  subsequent  amalgamation  in  pans.  But  if  those  metals  are 
intended  to  be  extracted  by  a  solving  process,  where  no  amalgamation 
takes  place,  the  gold  also  must  be  converted  into  a  chloride  while 
roasting.  Roasting  ores  in  which  gold  and  silver  is  present  with  salt, 
chloride  of  gold  is  formed,  according  to  Plattner;  but  before  the  ore 
becomes  red  hot,  the  gold  loses  a  part  of  its  chlorine,  is  reduced  to  a 
sub-chloride,  and,  at  a  little  higher  degree  of  heat,  to  metallic  gold. 

To  form  chloride  of  gold  by  way  of  roasting,  a  better  result  is  ob- 
tained in  the  furnace  if  the  ore  is  roasted  first  without  salt  till  the 
smell  of  sulphur  is  no  longer  perceptible;  and  then,  after  it  has  cooled 


ROASTING    OF    ORES.  43 

down  to  a  low  temperature,  the  salt  is  added,  and  the  whole  stirred 
for  some  time.  A  suitable  form  for  a  furnace  would  be  a  long  hearth 
or  furnace,  altered  in  such  a  way  that  the  second  hearth  should  be  ten 
twelve  inches  below  the  first;  the  arch  of  the  first  hearth,  however, 
should  continue  in  a  straight  line.  By  this  means  the  space  is.  widened 
and  the  temperature  brought  down  to  the  proper  degree.  The  ore  is 
charged  on  the  first  hearth  near  the  bridge,  and  roasted  in  the  usual  way 
oxidizing  the  sulphurets.  When  this  is  effected,  the  ore  is  shifted 
over  in  the  lower  furnace,  and  the  upper  charged  again.  As  soon  as 
the  roasted  ore  assumes  a  dark  red  heat,  the  salt  is  introduced  and 
raked  for  two  or  three  hours.  According  to  Roeszner,  a  combination 
of  gold  oxide  of  soda  and  chloride  of  sodium  (Au2  O3  Na  01)  is  formed. 
It  is  not  soluble  in  water,  and  but  slightly  in  salt  solution,  and  can- 
not be  amalgamated;  but  it  is  soluble  in  hyposulphite  of  soda  or  lime. 
V.  Lill  and  others  consider  the  gold  in  the  state  of  a  sub-chloride 
(Au  01.)  Hot  water  cannot  be  used  for  the  purpose  of  leaching  out 
base  metals,  as  the  chloride  of  gold  would  be  decomposed. 

Examples  of  Local  Roastings. 

Roasting  of  Silver  Ores  in  Freiberg  (Saxony.) 
The  amalgamation  process,  and  consequently  this  kind  of  roasting, 
was  given  up  long  ago  at  Freiberg;  but  the  method  of  roasting  as 
performed  there  is  nevertheless  very  interesting.  The  ore  subjected 
to  roasting  consisted  of  silver  glance,  brittle  silver  ore,  ruby  silver, 
metallic  silver,  fahl  ore,  bournonite,  zincblende,  sulphide  of  antimony, 
iron,  copper,  and  nickel  pyrites,  and  of  gangue,  quartz,  calc, 
brown,  heavy,  and  fluor  spar.  It  contained  from  sixty  to  one  hun- 
dred ounces  of  silver  per  ton. 

The  dry  crushed  ore  was  first  spread  on  a  platform ;  on  this  a  layer 
of  damp  ore,  from  the  wet  concentration,  was  laid,  and  then  10  per 
cent,  of  salt.  This  order  was  repeated  from  six  to  eight  times. 


44  ROASTING   OP    ORES. 

The  stratified  mass  was  mixed  thoroughly  by  means  of  shovels  and  a 
coarse  sieve.  This  mixture  contained  from  9  to  10  per  cent,  of  moist- 
ure. For  this  reason,  after  a  charge  of  450  to  500  pounds  was  in- 
troduced through  a  hole  in  the  roof,  the  fire  was  kept  very  low,  in 
order  to  try  it  at  a  dark  red  heat,  and  the  ore  was  diligently  raked 
by  two  men,  working  alternately.  •  As  soon  as  the  decrepitation  of 
thy  salt  ceased,  the  ore  was  ridged  from  the  bridge  toward  the  flue, 
through  the  middle  of  the  furnace,  and  the  formed  lumps  broken  up 
by  iron  hammers  attached  to  long  handles.  After  this  was  done,  the 
heat  was  increased,  whereby  the  ore,  under  constant  stirring,  assumed 
a  red  hot  condition,  and  the  sulphur  commenced  to  burn  quite  lively. 
This  stage  was  reached  in  two  hours  from  the  beginning.  The  desul- 
phurization  commences  with  the  burning  of  the  sulphur,  creating  a 
temperature  sufficiently  high  to  continue  the  roasting  without  fuel 
for  some  time.  Fumes  are  evolved,  consisting  of  steam,  antimony, 
sulphurous  acid,  arsenic,  etc.  This  desulphurization  takes  again  two 
hours,  the  workmen  all  the  time  raking  and  changing  the  hotter  part 
of  the  ore  at  the  bridge  with  the  cooler  at  the  flue.  The  temperature 
is  now  raised  to  a  light  red  heat;  the  ore  increases  in  volume,  emit- 
ting chlorides  of  metals,  chlorine,  hydrochloric  acid,  etc.  The  forma- 
tion of  the  chlorides  progresses  rapidly,  and  is  finished  in  three- 
quarters  of  an  hour.  The  charge  is  then  drawn  out.  A  too  long 
roasting  would  not  give  an  equally  good  result,  as  some  silver  might 
be  decomposed  to  the  metallic  state,  which  is  not  so  readily  amalga- 
mated as  the  chloride. 

\ 
Roasting  of  Argentiferous  Copper  Ores.     At  Arivaca  (Arizona)  the 

silver  ores  from  the  Heinzelmann  mine  consisted  principally  of  silver 
copper  glance(  stromeyerite)  with  51  per  cent,  of  silver;  fahl  ore  with 
from  2  to  15  per  cent,  silver,  containing  also  some  quicksilver;  zinc- 
blende,  galena  and  some  decomposed  argentiferous  copper  ores.  On  an 
average  the  ore  contained  from  $150  to  $200  per  ton,  and 


ROASTING    OF    ORES.  45 

from  10  to  15  per  cent,  of  copper.  After  crushing,  the  ore  was 
spread  on  a  platform  covered  with  8  per  cent,  of  salt,  and 
mixed  thoroughly  by  means  of  shovels.  Eight  hundred  pounds 
of  it,  as  a  charge,  were  introduced  through  the  roof  of  the 
furnace  (which  was  constructed  entirely  of  adobe),  spread  on  the 
hearth,  and,  at  a  dark  red  heat,  stirred  for  two  hours,  at  the  end  of 
which  time  the  flame  was  colored — intense  greenish-blue,  and  con- 
siderable fumes  were  emitted.  The  raking  continued  for  an  hour 
and  a  half  more  at  an  increased  heat,  and  during  this  time  the  ore 
was  moved  three  times  from  the  bridge  to  the  flue  and  back. 

A  sample  taken  from  the  furnace  at  this  time,  put  on  a  canvas 
filter,  wet  with  salt  solution  and  leached  with  a  hot  concentrated  so- 
lution of  salt,  gave  a  clear  liquid,  which,  diluted  with  water,  showed 
a  strong  white  precipitate  of  chloride  of  silver,  mixed  with  antimony 
and  lead;  but  the  quicksilver,  treated  with  the  same  sample  of  ore 
and  water,  was  cut  and  blackened  to  a  high  degree.  For  this  reason, 
from  5  to  6  per  cent,  of  pulverized  lime  was  thrown  into  the  furnace 
by  means  of  a  scoop,  as  much  as  possible  over  the  whole  surface  of 
the  ore,  and  then  raked  and  stirred  diligently  in  order  to  finish  the 
mixing  in  the  shortest  time.  After  four  hours  from  the  beginning, 
the  temperature  was  raised  to  a  light  red  heat  for  half  an  hour,  and 
the  roasting  was  finished.  There  were  yet  a  great  many  base  metal 
chlorides  in  the  ore,  but  as  metallic  copper  was  used  in  the  barrels, 
the  silver  turned  out  always  over  900  fine.  The  loss  of  silver  was 
12.5  to  13  per  cent. 

Roasting  of  Copper  Matte.  In  smelting  argentiferous  copper  ores, 
the  process  is  sometimes  regulated  to  produce  a  sulphide  of  copper, 
containing  silver  and  base  metals,  as  antimony,  arsenic,  zinc,  iron, 
etc.  This  sulphide  of  copper,  or  copper  matte,  was  roasted  formerly 
and  smelted  again  to  produce  black  copper;  that  is,  impure  metallic 
copper.  For  the  purpose  of  extracting  the  silver  therefrom,  the  cop- 


46  ROASTING   OF    ORES. 

per  was  melted  together  with  a  certain  percentage  of  lead,  and  the 
latter,  with  the  silver,  extracted  by  liquation  and  cupelled.  The 
remaining  copper  contained  still  some  silver  and  lead,  and  the  process 
was  a  very  lengthy  one  before  finished.  To  avoid  the  liquation, 
the  copper  matte  was  treated  by  amalgamation,  and  the  silver  ex- 
tracted at  once.  For  this  purpose  the  matte  was  crushed  and  sifted, 
and  the  coarse  part  ground. 

Of  this  powdered  matte,  300  pounds  are  charged  in  a  double  fur- 
nace, of  which  the  upper  hearth  prepares  the  ore  by  a  moderate 
roasting,  while  the  lower  one  finishes  the  operation  at  a  higher  tem- 
perature. In  each  of  the  hearth-departments  the  matte  is  treated 
for  two  hours  and  a  half.  Silver  and  the  other  metals  are  first  con- 
verted into  sulphates,  and  then  mostly  decomposed  to  oxides;  but  the 
silver  remains  for  the  greatest  part  in  metallic  condition.  The  matte 
is  drawn  out,  mixed  with  8  per  cent,  of  salt  and  12  per  cent,  of  lime, 
and  with  salt  water  into  a  paste,  which  is  allowed  to  rest  for  twelve 
or  fourteen  hours.  The  paste  is  then  dried,  powdered  between  rollers, 
and  again  roasted  two  hours  and  a  half.  During  this  process,  sam- 
ples are  taken  and  mixed  with  water  and  a  few  drops  of  mercury.  If 
this  appears  coated  bluish,  it  proves  the  presence  of  metallic  salts, 
and  some  more  lime  must  be  added;  if  after  this  the  quicksilver  re- 
mains perfectly  white,— parting,  however,  in  many  minute  globules, — 
it  proves  that  too  much  of  the  lime  was  used,  and  in  this  case  some 
of  the  first  roasted  matte  is  added. 

The  purpose  of  wetting  the  roasted  ore,  as  above  described,  is  the 
formation  of  chloride  of  silver.  As  there  are  always  sulphates  of  the 
metals  present  after  the  first  roasting,  they  decompose  the  salt,  and 
the  chlorine  acts  on  the  metallic  silver.  This  process  is  not  perfectly 
finished,  and  therefore  the  second  roasting. 

The  roasting  of  copper  matte  is  also  performed  differently  from  the 
above  method ;  that  the  temperature  in  the  upper  hearth  is  kept  so 


ROASTING   OF    ORES.  47 

low  as  is  sufficient  to  drive  out  the  sulphur.  Although  the  heat  is 
very  low,  the  stirring  of  the  red  hot  matte  is  important;  the  more 
diligent  the  stirring  is,  so  much  less  lumps  will  be  formed.  This  roast- 
ing on  the  upper  hearth  may  require  five  hours,  after  which  time  it  is 
drawn  on  the  lower  hearth,  and,  under  constant  stirring,  roasted  for 
two  hours  more  at  a  moderate  heat;  then  the  temperature  is  increased, 
and  the  roasting  continued  for  two  or  three  hours  more,  till  no  evo- 
lution of  fumes  is  perceived.  A  sample  taken  from  the  furnace  and 
leached  with  water,  should  color  the  filtered  solution  hardly  percepti- 
ble blue.  The  copper  is  now  mostly  turned  into  oxide;  also  the  iron. 
The  silver  is  converted  principally  into  a  sulphate,  and  if  there  is 
some  lead,  zinc,  arsenic,  and  antimony  in  the  matte,  then  these  metals 
are  found  to  be  sulphates,  arsonates  and  antimonates.  In  small  quan- 
tities these  metals  are  not  injurious;  but  lead  and  antimony,  if  too 
much  of  it,  are  troublesome. 

In  order  to  chloridize  the  silver,  about  five  per  cent,  of  salt  is  now 
introduced  into  the  furnace,  after  the  ore  is  cooled  down  a  little,  or  a 
part  of  it  is  taken  out  and  mixed  with  the  salt  on  the  floor,  charged 
back  in  the  furnace  and  mixed  with  the  balance  of  the  ore.  Increas- 
ing the  temperature,  the  salt  soon  commences  to  volatilize  and  chlori- 
dizes  directly,  and  partly  also  in  contact  with  some  sulphates.  Dur- 
ing this  chloridizing  roasting  the  temperature  is  kept  always  moderate 
till  to  the  end.  If  there  is  only  copper,  iron  and  silver  in  the  matte, 
all  these  metals  are  partly  chloridized,  the  first  two  assisting  the  chlo- 
rination  of  silver ;  but  if  the  roasting  before  the  addition  of  salt  was 
imperfect,  leaving  too  much  of  the  sulphates  undecomposed,  then  a 
great  deal  of  the  copper  volatilizes  as  chloride  of  copper,  and  causes 
at  the  same  time  also  a  loss  of  silver.  If  the  matte  contains  also  lead, 
zinc,  antimony  and  arsenic,  these  metals  volatilize  as  chlorides  and 
cause  a  still  greater  loss  in  silver  and  copper. 

Roasting  of  Black  Copper.  The  black  copper  obtained  from  smelt- 
ing (in  Schmoellnitz,  Hungary)  contains  from  110  to  150  ounces  of 


48  ROASTING    OF    ORES. 

silver  per  ton,  and  85  to  89  per  cent,  of  copper.  To  pulverize  this  it 
must  be  made  red  hot  in  a  reverberatory  furnace  and  crushed  while 
red  hot.  The  powder  must  be  sifted  and  then  ground  fine.  The 
pulverized  metal  is  then  mixed  with  7  to  9  per  cent,  of  salt,  and 
roasted  in  the  usual  way  for  from  six  to  six  and  a  half  hours,  in  charges 
of  400  pounds  each.  No  green  vitriol  is  added  for  the  purpose  of  de- 
composing the  salt ;  and  as  there  is  not  more  than  from  J  to  1  per 
cent,  of  sulphur  in  the  black  copper,  the  salt  decomposes  through 
direct  action  on  the  copper.  First,  chloride  and  sub-chloride  of  copper 
are  formed.  The  copper  chloride  transfers  chlorine  to  the  metallic 
silver,  and  is  reduced  to  a  sub-chloride. 

In  other  places  iron  pyrites  is  added  to  the  black  copper,  by  which 
the  chlorination  is  promoted.  At  Oravitza  (Banat)  5  per  cent,  of 
iron  pyrites  and  12  per  cent,  salt  are  used,  roasting  twelve  hours. 
The  loss  of  silver  is  7  per  cent.,  and  of  copper  3  per  cent.,  during  the 
roasting.  The  expense  of  roasting  is  $7.30  per  ton. 

Roasting  of  Silver  Ores  for  the  Patera  Process.  The  ores  treated 
by  Patera's  process  (page  128)  at  Joachimsthal  are  remarkable  for  the 
numerous  mineral  species  occurring  in  the  ore.  Among  these  may  be 
mentioned  silver,  lead,  different  compounds  of  copper,  bismuth,  iron, 
uranium,  nickel,  cobalt,  etc.,  sulphur,  arsenic  and  antimony.  Before 
the  introduction  of  Patera's  process,  the  extraction  of  silver,  on  ac- 
count of  so  many  base  metals,  was  very  difficult.  The  success  of 
Patera  was  not  so  much  in  adopting  hyposulphites,  as  proposed  by 
Percy  and  Hauch,  but  in  his  modification  of  the  roasting  process,  by 
which  only  the  silver  was  converted  into  a  chloride. 

The  pulverized  ore  is  placed  in  the  furnace,  in  charges  of  from 
four  to  five  hundred  pounds.  First  quite  a  moderate  heat  is  applied, 
and  gradually  increased,  but  not  so  much  as  to  induce  clotting.  As 
soon  as  the  ore  appears  red  hot,  steam  is  admitted,  with  about  four 
pounds  pressure  to  the  square  inch.  As  the  steam  consumes  heat, 


ROASTING   OF   ORES.  49 

more  fuel  must  be  used  to  keep  up  a  red  heat.  The  ore  must  be  con- 
stantly raked  during  the  whole  period  of  this  roasting.  It  takes  about 
four  hours  to  finish  this  process,  after  which  the  ore  is  drawn  and  per- 
mitted to  cool.  The  iron  appears  now  as  an  oxide — also  the  copper ; 
some  sulphate  of  copper  is  also  present,  and  the  silver  is  principally 
in  the  state  of  a  sulphate. 

The  oxidized  ore  is  now  ground  finer,  mixed  with  from  5  to  12  per 
cent,  of  common  salt,  and,  at  the  same  time,  with  2  to  3  per  cent,  of 
calcined  green  vitriol.  This  mixture  is  spread  upon  the  hearth  in  the 
furnace,  and. subjected  to  a  second,  now  chloridizing,  roasting.  It 
takes  about  an  hour  before  a  red  heat  is  reached.  Steam  is  then 
introduced,  as  above,  under  continuous  stirring.  The  fire  is  gradually 
increased,  and  the  roasting  finished  within  from  five  to  eight  hours, 
according  to  the  value  of  the  ore.  There  are  condensing  chambers 
for  catching  volatile  metals  and  ore  dust.  They  are  of  importance  if 
rich  ore  is  treated,  and  without  this  contrivance  several  per  cent,  of 
silver  would  be  lost.  When  finished,  the  ore  is  drawn  and  allowed 
to  lie  undisturbed  for  some  time,  after  it  has  been  moistened.  In 
this  condition  the  chlorination  continues.  The  application  of  steam 
causes  nearly  twice  the  consumption  of  fuel,  but  it  has  been  shown 
that,  by  the  steam,  hydrochloric  acid  is  formed,  whereby  arsenic  and 
antimony  are  expelled,  and  at  the  same  time  the  chlorination  of  the 
silver  greatly  favored.  Over  one  ton  of  coal  and  one-half  cord  of 
wood  are  consumed  for  each  ton  of  ore  roasted  in  this  way. 

Roasting  for  Augustus  Process.  To  this  process  principally  matte 
is  subjected.  The  process  requires  a  chloridizing  roasting.  The  ore, 
if  not  itself  rich  in  sulphurets,  is  mixed  with  iron  pyrites,  slag,  lime, 
etc.,  and  smelted.  The  molten  sulphide  of  iron  takes  up  the  silver 
and  deposits  itself  below  the  slag  on  the  bottom  of  the  furnace.  The 
silver  is  thus  separated  from  the  earthy  part  and  concentrated  in  the 
matte.  Argentiferous  copper  ores  are  likewise  smelted  for  the  pur- 


50  ROASTING   OF   ORES. 

pose  of  obtaining  argentiferous  copper  matte.  The  matte  is  then 
finely  pulverized,  and  400  pounds  of  it  are  introduced  into  the  fur- 
nace. The  roasting  goes  on  now  in  the  usual  way,  by  starting  with 
a  moderate  temperature,  gradually  increasing  it,  exposing  every  por- 
tion of  the  ore  to  the  intense  heat  by  frequent  stirring,  etc.  At  the 
end  of  eight  hours  the  roasting  is  generally  completed,  the  matte  looks 
dark  and  earthy,  and  no  fumes  of  sulphurous  acid  can  be  perceived. 
The  roasted  stuff  is  now  drawn  out  and  permitted  to  cool.  After  this 
the  matte  powder  is  ground  finer,  sifted  or  bolted,  and  given  back  to 
the  furnace. 

Four  hundred  pounds  are  mixed  with  5  per  cent,  of  salt.  Sulphates 
are  present,  but  oxides  of  metals  predominate  in  the  mixture.  The 
formation  of  the  chlorides  commences  immediately,  and  the  roasting 
is  concluded  after  oife  or  two  hours.  The  temperature  in  this  second 
roasting  is  kept  low,  as  the  smelting  of  the  chloride  of  silver  must  be 
prevented — for  when  melted  the  chloride  of  silver  dissolves  with 
more  difficulty  in  a  salt  solution.  In  other  places,  after  the  first 
roasting,  the  mass  is  not  taken  out  and  re-ground,  but,  when  finished, 
only  a  portion  is  drawn,  mixed  with  from  1  to  6  per  cent  of  salt, 
according  to  the  purity  of  the  matte,  charged  again,  mixed  with  the 
balance  of  ore  in  the  furnace,  and  roasted  for  one-half  to  three- 
quarters  of  an  hour. 

Roasting  of  Silver  Ores  in  a  Long  Furnace  at  San  Marcial,  Sonora, 
Mexico.  For  the  purpose  of  roasting  silver  ores  for  the  Solving  and 
Precipitation  Process,  there  are  several  long  furnaces  (thirty  feet 
long)  built  by  Mr.  O.  Hofmann,  in  Sonora.  Using  long  furnaces 
is  found  a  great  economy  in  every  respect.  A  great  advantage  results 
also  from  moving  the  ore,  at  intervals  of  from  one  to  three  hours, 
from  one  hearth  to  the  other.  By  this  means  it  is  impossible,  even 
with  careless  roasters,  to  find  raw  ore  in  the  finished  charge,  as  would 
happen  under  such  circumstances  if  the  corners  of  single  roasting  fur- 
naces were  not  very  carefully  attended  to  during  the  roasting. 


ROASTING    OP   ORES.  51 

The  furnace  at  San  Marcial  is  sixty  feet  long.  It  is  a  level  hearth 
sixty  feet  long,  representing  six  furnaces,  each  ten  feet  long,  parted 
only  by  the  projecting  wall  inside. 

There  are  six  working  doors  on  one  side,  and,  on  account  of  the  length, 
an  auxiliary  fire-place  is  placed  at  the  back  side,  before  the  last  two 
hearths.  Each  hearth  contains  800  pounds  of  ore,  and  is  attended  by 
two  Yaque  Indians  at  a  time,  stirring  alternately.  These  twelve 
Indians  perform  all  the  work  about  the  furnace,  carry  the  wood  from 
the  adjoining  yard,  split  what  is  too  thick,  carry  out  the  ashes,  etc. 
The  ore  on  the  first  hearth,  nearest  to  the  fire-place,  has  always  a 
light  red  heat,  which  decreases  with  the  distance  from  the  bridge,  so 
that  the  fifth  hearth  appears  quite  dark  if  not  assisted  by  the  second 
fire-place.  After  stirring  one  hour  on  all  six  hearths,  the  charge  from 
the  first  is  drawn  out  through  a  door  in  the  rear,  on  a  large,  smooth 
platform,  and  immediately  spread  by  means  of  shovels  in  a  layer  one 
or  one-and-a-half  inches  thick,  so  as  to  have  it  cool  enough  for  trans- 
portation to  the  sifting  apparatus  after  the  lapse  of  one  hour.  As 
soon  as  the  hearth  is  cleared,  the  ore  on  the  second  hearth  is  moved 
over  to  the  first,  then  that  on  the  third  to  the  second,  and  so  on,  till 
the  sixth  hearth  remains  empty  and  is  charged  through  the  funnel  in 
the  roof  with  a  new  charge.  It  will  be  seen  that  800  pounds  of  ore 
are  drawn  out  every  hour,  dhd  that  each  charge  is  exposed  to  the  fire 
for  six  hours.  It  is  thus  evident  that,  being  moved  six  times  from 
one  hearth  to  the  other,  the  ore  arrives  perfectly  prepared  to  the 
finishing  heat.  After  roasting  and  sifting,  the  ore  is  amalgamated  in 
pans,  but  as  it  contains  some  carbonate  and  sulphuret  of  lead  the 
amalgam  is  charged  with  base  metals,  so  much  that  refining  by  cupel- 
lation  is  necessary.  From  8  to  10  per  cent,  of  salt  is  added  and 
mixed  with  the  ore  before  it  is  charged.  Preparations  are  made  now 
to  introduce  the  solving  and  precipitation  process,  if  successful  on  that 
kind  of  ore. 

According  to  an  analysis  made  by  Mr,  Graff,  the  roasted  ore  from 


52  BOASTING    OF    ORES. 

single  furnaces  (treating  $100  ore)  contained  5  per  cent,  less  of 
chloride  of  silver  than  that  from  a  long  one.  Long  roasting  furnaces 
are  especially  adapted  for  roasting  sulphurets  containing  gold.  Con- 
centrated sulphurets,  or  ore  containing  an  abundance  of  sulphurets, 
allow  the  use  of  a  very  long  furnace,  with  only  one  fire-place,  on 
account  of  the  heat  created  by  the  burning  sulphur. 

The  roasting  expenses  at  San  Marcial,  with  the  furnace  sixty  feet 
long,  were  as  follows : 

24  men,  day  and  night,  at  50  cents $12  00 

2  cords  of  wood  at  $3 6  00 

8  per  cent,  of  salt=l,536  fts.  at  2  c 30  72 

Repairs,  etc 3  00 


Total  expenses  on  9.8  tons $51  72 

or  $5.27  per  ton. 

A  furnace  thirty  feet  long,  with  the  same  kind  of  laborers,  800- 
pound  charges,  drawing  every  two  hours, — that  is,  4.8  tons  in  24 
hours,  shows  the  following  expenses: 

8  roasters  at  50c $  4  00 

1^  cords  of  wood  at  $3 4  50 

8  per  cent,  of  salt  at  2^c 19  00 

Other  expenses : . . .     2  00 

Total $29  70 

or  $6.18  per  ton. 

B.    Oxidizing  Roasting. 

The  purpose  of  the  oxidizing  roasting  is  either  to  expel  volatile 
substances  which  are  combined  with  the  metals  (as  sulphur  or  arsenic), 
or  to  expel  volatile  metals  which  are  considered  obnoxious  to  further 
treatment  of  silver  ores  (as  antimony,  lead,  zinc,  etc).  The  oxygen 
has  a  large  share  in  this  transaction,  and  combines  with  the  volatile 
substances,  as  well  as  with  the  metals.  Some  of  the  combinations 
with  the  oxygen  become  volatile— as,  for  instance,  sulphurous,  arsenous 
and  antimonious  acids,  lea.d  a,i}d  zinc  oxides,  etc.  Other  combinations 


ROASTING   OF   ORES.  55 

again  are  not  volatile,  as  the  formed  sulphates,  arsenates  and  anti- 
monates.  Some  of  these  latter  compounds  can  be  disengaged  by  an 
increased  heat,  as  the  sulphates  of  iron  and  of  copper,  whereby  the 
sulphuric  acid  escapes,  while  the  remaining  metal  turns  into  an  oxide. 
Others  cannot  be  decomposed  by  an  increased  heat,  or  an  increased 
heat  is  considered  injurious  for  other  reasons ;  and  in  this  case  such 
combinations  may  be  decomposed  by  an  addition  of  charcoal  powder, 
saw-dust,  or  the  application  of  hydrogen.  The  sulphuric  acid  is 
reduced  by  the  carbon  to  sulphurous  acid,  and  goes  off,  and  so  also 
the  arsenic  and  antimony.  The  carbon  deprives  the  sulphate  or 
arsenate  of  a  part  of  its  oxygen,  and  escapes  as  carbonic  acid. 

What  Process  Requires  Oxidizing  Roasting. 

The  oxidizing  roasting  is  in  use  partly  as  a  preparatory  treatment 
for  a  chloridizing  roasting.  For  Ziervogel's  process  alone  an  oxidizing 
roasting  is  in  use.  For  amalgamation  of  silver  ore  it  is  not  suitable; 
but  it  is  important  with  the  smelting  processes,  and  also  in  extracting 
gold  from  gold  ores — principally  from  sulphurets  (iron  pyrites).  For 
this  purpose  long  furnaces  (Fig.  8)  are  the  most  suitable,  but  also 
double  furnaces. 

The  main  point  in  the  roasting  for  Ziervogel's  process  is  the  creation 
of  a  sulphate  of  silver,  and  the  oxidation  of  the  base  metals  as  far  as 
possible.  As  the  decomposition  of  sulphates  of  different  metals 
depends  on  different  degrees  of  temperature,  such  roasting  appears  of 
a  very  delicate  nature.  To  this  process  principally  argentiferous 
copper  matte  is  subjected. 

Roasting  of  Copper  matte.  When  pulverized  until  fine  enough  to 
pass  through  a  sieve  of  thirty-three  holes  to  the  running  inch,  the 
mass  is  introduced  into  the  furnace  and  spread  out  by  means  of  rakes. 
The  matte  inclines  much  to  clotting.  For  this  reason  a  very  moderate 


54  ROASTING   OF   ORES. 

temperature  is  applied,  more  for  drying  than  for  roasting.  The  matte 
is  left  quiet  for  about  fifteen  minutes,  after  which  the  stirring  is 
commenced  and  continued  without  stopping  for  an  hour.  During 
this  time  many  lumps  are  formed,  which  the  roaster  tries  to  crush  to 
powder.  Near  the  working  door  the  stuff  is  exposed  to  a  draft  of 
fresh  air,  in  consequence  of  which  the  roasting  on  that  place  pro- 
gresses more  rapidly  than  it  does  further  back.  This  makes  a 
shifting  of  the  stuff  necessary  after  one  hour's  roasting.  The  other 
roaster  now  takes  the  rake  and  stirs  the  matte  again  for  an  hour, 
doing  the  work  precisely  as  the  first  roaster  did.  The  roasters  change  in 
this  way  every  hour  for  from  five  and  one-half  to  five  and  three-quarters 
hours.  This  roasting  is  performed  on  the  upper  hearth  of  the  double 
furnace.  Twenty-five  pounds  of  coal  dust  are  mixed  with  the  matte, 
causing  an  ignition  and  emission  of  gases,  and  the  whole  mass  is 
transferred  to  the  lower  hearth  through  a  hole  in  the  bottom.  The 
upper  hearth  is  now  charged  with  500  pounds  of  matte  anew. 

The  sulphur  commences  to  burn  after  a  raking  of  three-quarters  of 
an  hour,  and  the  mass  increases  in  volume  when  the  hearth  is  covered 
about  four  inches  with  matte.  During  the  roasting  all  metals  are 
converted  into  sulphates,  of  which,  toward  the  end  of  the  operation, 
iron  and  cine  vitriol  are  decomposed,  leaving  those  metals  as  oxides. 
Copper,  nickel  and  cobalt  remain  in  the  state  of  sulphates. 

The  lower  hearth  is  in  a  light  red  hot  condition  when  the  matte 
falls  in  from  the  upper  hearth.  To  prevent  the  rapid  burning  of  the 
admixed  coal  dust,  and  the  clotting  of  the  mass,  a  vigorous  stirring 
for  an  hour,  with  closed  dampers,  is  strictly  observed.  The  stuff  is 
now  shifted  and  then  the  damper  opened.  There  now  follows  a  sharp 
oxidizing  roasting,  with  free  access  of  air,  for  one  hour  and  a  half. 
By  means  of  the  air  current,  the  roasting  mass  is  cooled  down  so  far 
that  it  appears  quite  dark.  To  see  the  progress  of  the  roasting,  a 
sample  is  taken  out  and  examined,  either  on  a  porcelain  dish  or  on  a 


BOASTING    OF    ORES.  55 

filter  with  cold  water.  The  leach  must  appear  of  a  clear  blue  color, 
and  an  addition  of  salt  solution  must  give  some  white  precipitate, 
proving  the  beginning  of  the  formation  of  sulphate  of  silver.  If  the 
nitrate  shows  a  greenish-blue  color,  the  presence  of  sulphate  of  iron 
is  apparent,  and  in  this  case  the  oxidizing  roasting  must  continue. 

The  purpose  of  the  addition  of  coal  dust  is  the  reduction  of  sul- 
phates to  basic  salts,  whereby  sulphurous  acid  is  emitted.  With  the 
opening  of  the  damper  the  oxidation  progresses,  the  sulphate  of  iron 
is  decomposed  almost  entirely,  the  sub-oxide  of  copper  turns  into 
oxide,  and  when  the  oxidizing  roasting  is  finished,  the  mass  contains 
mostly  oxides,  but  also  basic  salts.  There  are  copper,  iron  and  zinc 
oxides,  sulphates  of  copper  and  zinc,  while  the  silver  as  yet  consists 
principally  of  an  undecomposed  sulphide.  The  next  stage  of  roasting 
at  an  increased  temperature  is  the  last  one.  It  is  directed  toward 
the  sulphatization  of  the  silver  and  complete  oxidation  of  the  base 
metals.  It  takes  two  hours  and  a  half  to  accomplish  this  result,  under 
continuous  raking  and  increasing  the  temperature  to  a  light  red  heat. 
Samples  are  taken  again  as  before,  and  examined  in  the  same  way. 
The  leach  must  appear  only  of  a  bluish  tint,  and  on  adding  salt  solu- 
tion, a  heavy  precipitate  must  fall,  caused  by  chloride  of  silver.  The 
whole  roasting  period  on  the  lower  hearth,  as  on  the  upper,  takes 
from  five  and  one-half  to  five  and  three-quarters  hours. 

The  formation  of  the  sulphate  of  silver  in  the  last  period  at  an  in- 
creased heat,  is  due  to  the  sulphuric  acid  in  gaseous  form,  emanating 
from  the  sulphate  of  copper.  It  attacks  the  sulphide  of  silver,  and 
combines  with  it  to  a  sulphate.  Ninety-two  per  cent,  of  the  silver  is 
extracted  after  this  roasting.  If  in  the  last  period  the  feeding  with 
fuel  should  be  carelessly  performed,  so  as  to  give  a  smoky  flame,  some 
copper  oxide  will  be  reduced  to  sub-oxide,  and  this  will  precipitate 
metallic  silver  while  leaching,  causing  a  loss.  If  the  roasting  should 
not  continue  long  enough,  some  sulphide  remains  undecomposed;  and, 


56  ROASTING   OF   ORES. 

on  the  other  hand,  if  the  roasting  should  last  too  long,  a  part  of  the 
sulphate  of  silver  would  be  decomposed  to  metallic  silver  and  could 
not  be  leached  out.  These  circumstances  show  that  this  kind  of  roast- 
ing demands  a  great  deal  of  attention,  in  order  to  obtain  a  perfect 
result.  The  temperature  on  the  lower  hearth  in  the  beginning  is  500 
to  550  degrees  Centigrade;  it  sinks  then  to  425,  and  rises  again  at  the 
end  of  the  operation  to  770  degrees. 

Roasting  of  Iron  Sulphurets  Containing  Gold,  and 
of  Tellurides  of  Gold. 

The  gold  is  generally  found  in  a  free  state  as  metallic  gold.  In  this 
state  it  is  easily  extracted  by  proper  amalgamation.  Often,  however, 
the  gold  is  combined  with  other  substances,  so  that  amalgamation  is 
of  no  avail  unless  the  gold  is  set  free  by  roasting.  Iron  pyrites  and 
arsenical  pyrites  are  the  principal  ores  containing  the  gold  in  a  condi- 
tion unfit  for  direct  amalgamation.  Also  telluride  of  gold  must  be 
subjected  to  roasting  before  amalgamation. 

It  is  not  decided  whether  the  gold  in  sulphurets  exists  in  a  metallic 
condition,  finely  impregnated,  or  whether  it  is,  like  the  iron  of  the 
pyrites,  chemically  combined  with  the  sulphur.  Some  believe  because 
after  the  free  gold  is  extracted,  and  the  sulphurets  still  finer  pulver- 
ized, some  more  free  gold  appears,  that  for  this  reason,  all  the  gold  in 
the  sulphuret  can  be  set  free  if  only  a  sufficiently  fine  grinding  is 
effected.  This  conclusion,  however,  does  not  seem  to  be  correct.  The 
sulphurets  may  contain  free  gold,  so  minute  that  after  each  grinding 
some  free  gold  is  exposed,  but  for  all  this  there  may  be  a  great  deal  of 
it  chemically  combined,  which  never  can  be  set  free  by  trituration. 
There  are  sulphurets  in  California  assaying  $1200,  and  more,  per  ton, 
and  they  may  be  ground  to  the  very  finest  sloam,  but  no  free  gold,  or 
only  very  little,  will  be  visible  under  a  good  microscope.  This  seems 
to  confirm  the  theory  of  a  chemical  combination  much  more,  than  the 


ROASTING   OP   ORES.  57 

presence  of  some  free  particles  of  gold  after  a  finer  trituration,  which 
proves  only  that  free  gold  is  in  sulphurets,  but  not  that  there  is  no 
chemical  combination  besides  the  free  gold.  The  roasting  of  the 

Telluride  of  Gold 

or  of  telluride  of  silver  rich  in  gold  (Petzite)  is  a  most  delicate  opera- 
tion. Not  all  tellurium  combination  with  gold  are  losing  the  gold 
to  a  notable  extent  while  roasting,  but  some  do,  and  that  to  a  consid- 
erable amount,  up  to  20  per  cent.,  perhaps  even  more.  The  loss  is  no 
mechanical  one  occasioned  by  the  draught  of  the  furnace,  but  princi- 
pally by  volatilization.  This  occurs  sometimes  at  a  very  low  heat, 
As  soon  as  the  ore  gets  hot  enough  to  show  rising  fumes,  it  is  advisa- 
ble to  take  a  sample  and  assay  it  and  not  to  increase  the  temperature 
until  the  result  of  the  assay  is  known. 

As  an  example  in  what  singular  condition  the  gold  is  in  some  com- 
binations with  tellurium  is  the  Petzite,  containing  24.80  per  cent,  of 
gold  and  40.60  of  silver.  Heating  a  small  piece  of  this  mineral  on  a 
piece  of  charcoal  or  on  the  blade  of  a  knife  to  tfre  temperature  of  about 
323  deg.  6',  the  melting  point  of  lead,  the  bluish  gray  mineral  turns  sud- 
denly yellow,  without  changing  its  shape.  Under  the  microscope  this 
yellow  color  is  found  to  be  due  to  innumerable  perfect  gold  globules 
which  cover  the  surface.  The  gold  separates  from  the  silver  and  tel- 
lurium in  a  molten  condition,  while  the  other  two  metals  remain  un- 
changed. The  gold  requires  1200°  6'  to  become  liquid,  and  in  this  case 
;  melts  even  below  323. 

If  salt  is  present  during  the  roasting,  the  chloride  of  tellurium  vola- 
tilizes voluminously,  and  it  is  possible  that  under  this  condition  the 
tellurium  causes  the  gold  to  volatilize  likewise.  The  author,  by  his 
own  experience — having  4  per  cent,  of  salt  in  the  ore  on  account  of 
silver — found  the  loss  of  gold  amounting  to  8  per  cent,  before  the  ore 
was  properly  red  hot.  This  was  two  hours  after  charging.  Under 


58  ROASTING   OP   ORES. 

this  circumstance  the  continuation  of  roasting  was  impossible ;  the  ore, 
therefore,  was  drawn  out  and  then  chloridized  with  chlorine  gas,  and  a 
good  result  obtained;  not,  however,  without  lixiviating  for  a  while 
with  chlorine  water  after  the  largest  part  of  the  gold  was  first  extracted 
after  chlorination  with  gas.  Whether  the  above  loss  would  have 
occurred  in  such  proportion  if  no  salt  had  been  added,  cannot  be  stated. 
It  is  indispensiblc,  then,  to  have  condensing  chambers  connected  with 
the  roasting  furnaces,  if  tellurides  have  to  be  treated,  in  order  to  con- 
dense the  volatilized  metals. 

In  Transylvania  (Hungary)  the  tellurium  ores  are  subjected  to 
Coasting,  then  smelted  for  matte,  the  matte  granulated,  treated  with 
diluted  sulphuric  acid,  and  the  residue,  containing  gold,  silver  and 
lead,  fused  with  lead  ores,  and  the  resulting  lead  cupelled.  In  Colo- 
rado the  tellurides  are  also  matted  with  other  ores,  the  matte  pulver- 
ized and  roasted  oxidizing  after  Zervogel's  method  to  convert  the 
silver  into  a  sulphate,  which  is  leached  out  with  water,  then  the  residue 
fused  again  to  black  copper,  which  contains  the  gold. 

At  Zalathna,  Transylvania,  the  gold  telluride  is  fused  direct  with 
lead  in  cupell  furnaces,  but  the  tellurium  in  this  case  is  lost.  If 
matted  with  iron  and  copper  sulphurets,  and  the  silver  extracted  by 
leaching  after  roasting,  the  gold  is  separated  from  the  copper  by  diluted 
sulphuric  acid,  producing  marketable  sulphate  of  copper. 

The  roasting  of  iron  sulphurets  is  a  very  simple  oxidizing  process. 
The  main  object  is  to  effect  a  perfect  desulphurization  in  order  to  set 
free  the  gold.  Generally,  no  loss  of  gold  is  suffered  during  the  roast- 
ing, neither  with  iron  pyrites,  nor  with  the  arsenical  pyrites,  although 
exposed  to  a  considerable  heat  and  for  a  long  time  (from  24  to  48 
hours).  Many  manipulators,  however,  who  have  had  much  experience 
in  roasting  of  gold  sulphurets,  complain  of  considerable  loss  of  gold 
with  some  kind  of  sulphurets,  which  they  could  not  avoid  in  spite  of 


ROASTING   FURNACES.  59 

all  experiments  made  with  reference  to  heat  and  general  treatment. 
It  is,  therefore,  always  advisable  to  investigate  the  loss  of  gold  with 
new  sulphurets  by  assay. 

The  simplest  way  to  ascertain  whether  the  loss  occurs  in  roasting  or 
otherwise — because  the  loss  may  happen  also  in  the  subsequent  treat- 
ment and  be  wrongfully  charged  to  roasting — is  to  weigh  out  one-half 
ounce  of  the  well  sampled  and  dried  sulphurets  before  charging  the 
furnace,  then  another  half-ounce  after  roasting.  But  the  sulphurets, 
after  roasting,  weigh  a  great  deal  less  than  before,  having  exchanged 
the  heavier  sulphur  for  the  lighter  oxygen,  consequently  the  gold 
appears  concentrated  in  the  smaller  weight  of  the  ore.  For  this  reason 
it  would  be  improper  to  take  half  an  ounce  of  the  concentrated  and 
compare  it  with  that  of  the  original  not  concentrated  sample.  In  this 
case  if  half  an  ounce  of  the  roasted  ore  were  assayed,  the  number  of 
ounces  would  come  out  higher,  and  the  actual  loss  either  covered,  or 
the  gold  would  seem  to  increase  in  the  furnace.  If  salt  is  used  in 
roasting  sulphurets,  it  is  generally  charged  in  the  last  hour,  and  the 
sample  should  be  taken  just  before  the  salt  is  added,  and  only  so  much 
for  the  intended  half  ounce,  as  found  after  roasting  of  half  an  ounce  as 
described  (page  34.) 

The  weighed  sample  of  the  raw  sulphurets  is  introduced  into  No.  8 
French  crucible,  in  which  a  mixture  of  one  ounce  of  soda  and  one 
ounce  of  litharge  was  previously  placed,  and  then  carefully  mixed  with 
a  glass  or  iron  rod.  After  this,  a  20-penny  nail  is  inserted  into  the 
mixture  and  the  whole  covered  with  about  one  ounce  of  borax.  The 
other  sample  of  roasted  ore  is  also  introduced  into  another  crucible, 
having  in  addition  to  the  soda  and  litharge  mixture  eight  grains  of 
powder  of  charcoal,  well  mixed  and  covered  with  about  one-half  ounce 
of  borax  in  small  pieces,  no  nail  being  required.  Both  crucibles  are  ex- 
posed to  a  good  heat  in  a  furnace  for  15  minutes,  the  covers  taken  off, 
the  nail  with  the  cupell  tongs  carefully  taken  out,  giving  a  gentle  jerk 


60  ROASTING    OP    OEES. 

before  the  nail  is  out  of  the  crucible,  the  contents  poured  out  and  the 
buttons  cupelled.  Using  the  nail  as  above,  the  time  which  would 
have  been  consumed  in  roasting  is  saved  and  the  result  perfectly  relia- 
ble ;  but  samples  of  arsenical  pyrites,  copper  sulphurets  or  iron  pyrites 
containing  arsenical  pyrites,  must  be  subjected  to  roasting. 

Iron  sulphurets,  well  concentrated,  carrying  only  an  admixture  of 
quartz,  require  only  a  perfect  oxidizing  roasting  in  order  to  give  a 
satisfactory  result  in  the  subsequent  chlorination ;  but  if  lime,  talc  or 
clay  is  with  the  sulphurets,  an  addition  of  salt  in  small  quantities, 
twenty  to  forty  pounds  to  the  ton,  is  necessary,  otherwise  the  precipi- 
tation of  gold  is  sometimes  very  troublesome,  and  as  the  presence  of 
salt  is  not  injurious  it  is  always  better  to  use  some  salt  with  the  sul- 
phurets. The  quantity  of  salt  must  be  increased  if  silver  should  be 
present.  For  the  roasting  of  sulphurets,  the  reverberatory  furnaces 
are  almost  exclusively  in  use,  and  of  these  again  long  ones,  with  two 
or  three  hearths,  also,  one  hearth  above  the  other.  These  last  furnaces 
have  the  advantage  that  the  floor  is  heated  from  below,  and  the  burn- 
ing of  sulphurets  on  the  upper  hearth  comes  sooner  in  heat.  There 
is  also  a  better  mixing  of  more  and  less  ignited  parts,  in-  drawing  the 
charge  to  the  lower  hearth,  and  the  roasting  can  be  performed  by  one 
man  as  well  as  in  the  long  furnace,  unless  the  sulphurets  are  of  such 
a  nature  that  the  roasting  is  finished  in  a  much  shorter  time  than 
usually. 

The  roasting  hearth  is  generally  10  feet  by  10,  or  12  by  12,  receiv- 
ing one  ton  of  sulphurets.  After  the  furnace  has  been  heated  up,  the 
sulphurets  are  introduced  and  spread  over  the  hearth,  and  the  fire 
kept  up  lively  enough  to  bring  the  sulphurets  in  a  red  hot  condition. 
After  this  it  takes  very  little  fuel,  as  the  burning  is  maintained  by 
the  sulphur  itself.  Nearly  half  of  the  sulphur  is  expelled  by  this  low 
heat.  On  exposing  a  fresh  surface  of  the  mass  by  stirring,  the  burn- 
ing of  the  sulphur  with  a  bluish  flame  can  be  seen  distinctly.  The 
hoe  is  principally  used  for  stirring.  It  must  be  as  light  as  possible, 


ROASTING   OF   ORES.  61 

seven  to  eight  feet  long,  if  prepared  to  work  from  both  sides  of  the 
furnace.  The  stirring  is  performed  at  intervals  of  ten  to  fifteen  min- 
utes, and  whenever  the  circumstances  permit  two  roasters  to  be  em- 
ployed, the  time  of  roasting  will  be  shortened.  Oxidizing  roasting 
requires  more  stirring  than  the  chloridizing. 

In  proportion  as  the  oxidation  of  the  sulphurets  draws  nearer  to 
the  end,  the  temperature  decreases,  and  it  is  then  necessary  to  use 
more  fuel  to  keep  the  mass  at  a  good  red  heat.  It  takes  from  twenty 
to  forty  hours  before  the  roasting  of  one  charge  in  a  single  furnace 
may  be  considered  finished.  If,  in  throwing  up  sulphurets  in  the 
furnace  by  means  of  a  shovel  or  hoe,  many  brilliant  sparks  appear, 
this  denotes  that  the  roasting  is  not  finished,  but  must  be  continued 
till  this  appearance  ceases. 

In  a  long  furnace,  (Fig.  8)  the  hearth  near  the  bridge  is  always 
kept  at  a  bright  heat.  One  man  attends  to  the  ore  on  the  first 
hearth,  and  the  other  two  or  three  hearths  can  be  managed  by  a 
second.  In  moving  the  ore  from  one  hearth  to  the  other,  or  in  draw- 
ing the  charge  from  the  finishing  hearth,  these  two  men  assist  each 
other.  The  finishing  hearth  receives  the  ore  already  desulphurized  to 
a  great  extent,  containing  only  a  small  part  of  undecomposed 
sulphurets,  but  more  of  sulphates.  With  a  lively  heat  and  active 
stirring  at  intervals,  all  base  metals  ought  to  be  converted  into  oxides 
after  ten  or  twelve  hours.  The  heat  must  be  increased  to  a  bright  red 
heat,  but  not  to  a  white  heat,  else  the  gold  particles  would  melt, 
which,  with  a  good  magnifying  glass,  can  be  easily  detected  after 
washing  off  the  iron;  the  gold  appears  then  in  minute  globules,  the 
chlorination  of  which  is  more  difficult. 

Taking  a  sample  of  the  sulphurets  at  the  end  of  the  roasting,  and 
placing  in  a  filter  and  leaching  with  a  little  water,  the  leached  liquid 
will  contain  the  soluble  metals,  and  if,  by  addition  of  sulphide  of  soda 


62  ROASTING   FURNACES. 

or  of  lime  (page  33),  a  thick  precipitation  is  observed,  the  roasting 
should  continue  longer.  The  mixing  of  sulphurets  with  20  or  30  per 
cent,  of  sand  depreciates  the  value,  consumes  more  wood  in  roasting, 
requires  more  chlorine  gas,  more  handling;  but  there  is  no  evidence  of 
an  advantage  in  so  doing,  unless  a  better  result  is  obtained  by  having 
more  chlorine  gas  on  less  sulphurets,  the  volume  of  which  is  increased 
by  admixture  of  sand;  but  in  this  case  it  would  answer  better  if  the 
chlorine  gas  (treating  pure  sulphurets)  should  be  allowed  to  pass 
through  the  same  for  an  hour  after  it  appeared  on  top  of  the  charge. 
There  is  no  danger  of  backing  or  matting  of  pure  iron  sulphurets  if 
a  light  red  heat  is  applied,  unless  the  temperature  should  be  raised  to 
a  white  heat,  but  this  were  beyond  the  limit  of  roasting  heat. 

Roasting  Furnaces. 

Roasting  not  only  requires  much  care,  but  it  is  also  an  expensive 
operation.  For  this  reason  the  choice  of  the  right  kind  of  furnaces 
is  of  very  great  importance,  and  so  much  the  more  as  a  perfect  and 
economical  extraction  of  silver  depends  principally  on  the  result  of 
roasting.  The  chloridizing  roasting  is  known  to  be  the .  most  suitable 
way  for  the  subsequent  extraction  of  silver  in  whatever  way  it  may 
be  performed,  by  amalgamation  or  solving;  consequently  those  fur- 
naces in  which  the  ore  particles  are  exposed  to  the  action  of  chlorine 
and  other  chloridizing  gases  to  the  most  advantage,  must  be  considered 
the  best.  The  old  style  of  furnace  was  four  to  six  feet  wide  and  ten 
feet  long,  and  in  them  a  small  part  of  the  ore  was  exposed  to  the 
greatest  heat  near  the  bridge.  The  gases  evolved  were  carried  along 
by  the  draft,  being  in  contact  with  the  surface  of  the  ore  for  a  length 
of  ten  feet  while  passing  over  it;  but  on  account  of  the  narrowness 
of  the  hearth,  the  ore  at  the  bridge  had  to  be  changed  often  with  the 
cooler  part  at  the  flue. 

The  next  step  in  improvemont  of  reverberatory  furnaces  was  the 
adoption  of  wider  hearths,  even  wider  than  long.  The  heat  was  more 


ROASTING   FURNACES.  63 

uniform  and  the  result  better.  In  both  kinds  of  furnaces  the  chlo- 
rination  of  the  metals  depends  principally  on  the  chlorine  developed 
in  the  mass  of  the  ore  while  passing  through  it;  but  once  above  the 
surface,  the  chlorine  and  volatile  chloride  metals  have  less  chance  to 
transmit  their  chlorine  to  the  ore,  and  this  only  through  the  chlorina- 
tion  period.  During  two  or  three  hours  of  each  charge,  when  desul- 
phurization  and  sulphatization  are  going  on,  this  must  be  performed 
by  the  oxygen  of  the  air,  while,  if  chlorides  were  present  from  the 
beginning,  sulphurets,  sulphates  and  oxides  would  have  been  partly 
decomposed  directly  by  the  chlorine,  whereby  time  and  a  certain 
percentage  'of  salt  are  saved. 

In  this  respect  a  great  advantage  is  gained  by  the  introduction  of 
"long  furnaces,"  in  which  a  continual  formation  of  chlorides  on  the 
finishing  hearth  near  the  bridge  is  going  on,  volatile  chlorides  and  free 
chlorine  being  evolved,  which,  on  their  way  to  the  flue,  are  constantly 
in  contact  with  the  ore  for  a  space  of  thirty  or  fifty  feet  in  length. 
These  furnaces  show  a  great  economy  in  fuel,  labor  and  salt,  and  the 
roasted  ore  contains  a  better  percentage  of  chloride  of  silver. 

The  reverberatory  furnaces,  although  combining  many  valuable 
properties,  have  the  great  disadvantage  of  requiring  much  labor  to 
perform  the  roasting,  which,  in  this  country,  in  remote  localities, 
amounts  to  four  dollars  and  five  dollars  a  day  per  man.  In  order  to  re- 
duce this  heavy  expense,  mechanical  furnaces  of  various  construction 
have  been  introduced  with  great  success,  but  often  there  are  local  circum- 
stances which  favor  the  adoption  of  reverberatory.  A  very  important 
improvement  in  the  way  of  chloridizing  roasting  is  found  in  the 
Stetefeldt  and  some  other  furnaces,  where  all  ore  particles  are  involved 
in  chloridizing  gases  under  very  favorable  circumstances.  The  roast- 
ing is  cheap,  and  from  twenty  to  thirty  tons  of  ore  are  roasted  in 
twenty-four  hours. 

The  roasting  furnaces  do  not  require  a  white  heat,  hence  common 
bricks  can  be  used ;  but  it  is  nevertheless  advantageous 


64  ROASTING    FURNACES. 

if  the  fire-place  above  the  grates  is  built  of  fire  bricks. 
In  new  or  unpopulated  districts  even  unburned  bricks  or  adobe 
may  be  used;  they  stand  just  as  well  as  burned  bricks  of  the 
same  material,  except  in  the  floor  of  the  furnace,  which  is  worked 
out  in  two  or  three  months.  Hard  bricks  are  the  best  material  for 
the  hearth-floor,  placed  edgeways  (four  inches  high)  with  as  little  clay 
between  as  possible,  and  laid  carefully  and  well  fitting,  so  as  to  form 
a  level  and  smooth  surface.  All  parts  exposed  to  heat  must 
be  built  with  loam  or  clay,  not  with  mortar.  Many  masons  have  the 
custom  of  laying  three  heights  of  bricks  so  that  the  eight-inch  wall 
is  formed  by  two  rows  lengthways,  and  only  the  fourth  height  is  put 
cross  ways.  It  is  a  quick  work  and  may  answer  for  buildings,  but 
should  not  be  allowed  with  furnaces  where  the  expansive  heat  must 
be  considered,  especially  in  the  fire-place.  Each  alternate  row  of 
bricks  must  be  laid  crossways  to  the  preceding;  also  adjusting  the 
wall  with  the  hammer,  to  make  it  perpendicular  and  square,  after 
several  bricks  are  laid,  is  injurious.  The  outside  appearance  of  a 
furnace  is  of  minor  importance,  and  the  mason  must,  contrary  to  his 
general  idea,  pay  the  most  attention  to  the  solid  and  particular  work 
inside.  The  distance  of  the  arch  from  the  hearth  is  from  twenty-six 
to  thirty  inches  in  the  highest  point,  not  far  from  the  bridge ;  in  a  long 
furnace,  however,  the  roof  of  the  first  hearth  can  be  higher  from  the 
floor  by  four  to  five  inches,  according  to  the  length.  An  eight-inch 
thickness  of  the  arch  is  sufficient,  and  the  bricks  laid  with  the  eight- 
inch  side  perpendicular  form  a  more  durable  arch  than  one  of  twelve 
inch  thickness  composed  of  eight  and  four-inch  sides  of  the  bricks. 
The  furnace  must  be  secured  against  expansion  by  wall-braces  of 
cast-iron,  tightened  with  iron  rods  from  five  to  sixth-eighths  of  an  inch  in 
diameter.  The  rods  placed  over  the  length  of  the  furnace  are  stronger 
— one  inch  in  diameter.  In  place  of  wall-braces  also  wooden 
posts,  six  by  eight  inches,  are  used,  tied  by  iron  rods  on  the  top.  The 
lower  ends  are  generally  put  in  the  ground,  but  it  is  preferable  to  use 


ROASTING    FURNACES.  65 

rods  on  both  ends.  In  case  of  need,  even  the  rods  are  replaced  by 
timber.  For  the  passage  of  the  rods,  square  holes  must  be  provided 
in  the  masonry ;  also  for  the  escape  of  dampness  such  passages  are 
necessary  at  different  points,  especially  if  the  whole  block  consists  of 
masonry.  The  floor  of  the  hearth  should  be  three  feet  and  a  half 
above  the  ground ;  if  lower,  it  is  inconvenient  for  the  roaster. 

There  are  two  principal  classes  of  furnaces — such  as  are  managed 
by  hand  and  such  as  employ  machinery.  For  the  first  class,  mostly 
reverberatory  furnaces  are  in  use.  Of  the  second  class,  the  most  im- 
portant are  the  cylindrical  and  vertical  roasting  furnaces,  and  of  the 
last  ones  only  the  Stetefeldt  furnace. 

A.    Roasting  Furnaces  Managed  by  Handwork. 

Reverberatory  Furnaces.  Reverberatory  furnace  is  the  name  ap- 
plied to  all  horizontal  hearth  furnaces  provided  with  grates  and  fire- 
place on  one  side,  and  a  flue  connected  with  a  chimney  on  the  other. 
The  draft  here  is  created  by  the  chimney  instead  of  by  bellows,  as  in 
blast  furnaces ;  therefore  only  such  fuel  is  used  which  gives  a  flame, 
and  consequently  no  charcoal,  coke,  or  anthracite  is  serviceable  unless 
in  a  gas  reverberatory  furnace,  where  gas  (carbonic  oxide)  is  produced 
from  charcoal  or  other  fuel — sometimes  also  by  the  aid  of  compressed 
air — and  burned.  The  reverberatory  roasting  furnaces  are  constructed 
in  various  ways.  There  are  single  furnaces,  with  but  one  hearth,  and 
double  furnaces  with  two  hearths,  one  above  the  other.  Sometimes 
above  the  second  hearth  there  is  a  third  one  for  the  purpose  of  dry- 
ing the  charge.  Long  furnaces  are  preferable. 

A  Single  Roasting  Furnace  is  represented  by  Fig.  2,  showing  the 
section,  and  Fig.  3  the  ground  plan.  The  bottom,  «,  or  the  hearth, 
is  made  of  the  hardest  bricks,  laid  edgewise  and  as  close  as  possible. 
Some  masons  lay  the  bricks  flat.  This  mode  is  cheaper  and  quicker, 
but  far  inferior  and  less  durable  than  the  former  way,  and  requires  a 


66 


ROASTING    FURNACES. 


more  carefully  prepared  foundation.  The  very  best  bricks  must  be 
selected  for  the  hearth,  b  shows  the  discharge  hole  in  front  of  the 
hearth.  It  is  more  convenient  to  draw  the  ore  toward  the  front  hole 
than  to  have  a  door  for  this  purpose  behind,  but  circumstances  may 
decide  for  such  discharge  doors.  The  flue,  e,  is  in  connection  with 
the  flue-holes,  e,  in  the  arch,  as  indicated  by  dotted  lines  in  Fig.  3,  and 
is  from  nine  to  ten  inches  in  diameter.  The  flue-holes  in  the  arch 
have  the  advantage  that  no  ore  can  enter  when  being  stirred,  as  often 
happens  when  the  flue  commences  at  the  hearth.  The  distance  be- 
tween arch  and  hearth  near  the  bridge  is  twenty  to  twenty-one  inches 


ROASTING  FURNACES,  67 

and  near  the  flue  only  eight  inches.  The  flue  leads  into  the  chimney 
in  any  suitable  direction,  either  directly  or  through  a  dust  chamber. 
Often  the  flue  is  led  under  the  floor  (when  the  chimney  is  at  some 
distance  from  the  furnaces),  and  is  made  wide  enough  to  serve  as  a 
dust  chamber — say  two  feet  wide  and  three  feet  high,  or  wider  if 
several  furnaces  are  connected  therewith.  The  chimney  is  from 
twenty  to  fifty  feet  high,  and  from  one  and  one-half  to  three  or  four 
feet  square  in  the  clear.  On  the  top  of  the  chimney  an  iron  cover, 
controlled  by  a  chain,  regulates  the  draught.  This  is  practicable  only 
when  but  one  furnace  is  attached  to  the  chimney,  otherwise  dampers 
must  be  provided  for  each  furnace  in  the  flue.  The  bridge,  i,  is  much 
exposed  to  injury  by  fire  on  one  side,  and  by  raking  on  the  other ;  it 
is  therefore  advantageous  if  the  upper  part,  or  the  whole  bridge,  can 
be  made  in  two  or  three  parts  and  of  some  fire-proof  stone — sand- 
stone, granite,  or  some  conglomerate,  which  does  not  burst  when 
heated.  The  grates,  k,  are  twelve  to  sixteen  inches  below  the  top  of 
the  bridge,  eighteen  inches  wide,  and  from  six  to  seven  feet  long.  The 
space  between  the  grate-bars  is  one-fourth  to  one-half  of  an  inch. 

In  the  roof,  near  to  the  bridge,  is  an  opening  four  to  five  inches 
square,  of  cast  iron,  in  connection  with  a  funnel,  I,  of  sheet  iron. 
This  funnel  must  be  large  enough  to  receive  one  charge  of  the  ore. 
A  slide  keeps  the  ore  in  the  funnel.  The  roof  must  be  either  eight 
inches  thick,  or  the  double  length  of  the  brick;  that  is,  16  inches/ 
Under  the  hearth  there  is  an  arched  space,  d,  into  which  the  roasted 
ore  is  drawn  through  the  discharge  hole,  b,  either  directly  into  an 
iron  car  or  on  an  inclined  floor,  on  which  the  ore  slides  from  under- 
neath the  furnace.  In  front  this  space  is  shut  up  by  brick  work. 
For  the  purpose  of  easy  drying  it  is  well  to  have  open  some  holes,  g, 
for  the  escape  of  dampness.  It  is  not  necessary  to  build  the  block 
under  the  hearth  solidly  of  bricks.  The  space  inside  is  generally 
filled  up  with  rubbish  of  bricks  and  stone,  or  it  is  still  better  to  build 
the  floor  on  arches. 


68 


ROASTING  FURNACES. 


The  working  door,  o,  is  from  twenty-five  to  thirty  inches  wide.  In 
front  of  it  is  an  iron  roller  for  easier  handling  of  the  heavy  tools. 
The  door  is  eight  to  nine  inches  high.  The  cast  iron  door  frame,  for 
the  fire-place,  is  from  nine  to  twelve  inches  square.  When  completed, 
the  furnace  is  tied  by  iron  rods,  ny  both  ways.  The  uprights  are 
often  wooden  ones,  six  by  six  or  five  by  eight  inches.  In  place  of  the 
usual  iron  roller  in  front  of  the  working  door  a  far  better  contrivance, 
which  allows  the  hoe  easily  to  be  directed  to  the  required  points, 
which  is  not  the  case  with  the  roller  bars,  is  the  following,  illustrated 
by  the  engraving,  Fig.  4.  The  wheel,  a,  about  2J  inches  in  diameter 
with  J  inch  journals,  has  half  hollowed  face  to  receive  the  stem  of  the 
hoe.  This  wheel  rests  in  a  frame,  the  sides  of  which  are  forked  to  re- 
ceive the  journals  of  the  wheel,  and  ends  in  a  pin,  e,  which  is  inserted 
in  a  corresponding  hole  of  the  doorframe,  d,  Fig.  5.  This  can  easily  be 
attached  to  doorframes  already  fixed  on  the  furnace,  from  outside,  by 
drilling  two  holes,  and  then  to  screw  on  a  piece  of  flat  iron  in  the 
shape  as  seen  in  Fig.  5.  The  hoe  moves  easily  to  and  fro  on  the  wheel 
at  the  same  time  with  the  turn  of  the  fork  to  any  required  direction 
of  the  hearth.  Fig.  4  represents  one  quarter  of  the  natural  size. 

Hg.  4.  Fig.  5. 


It  is  very  important  to  dry  the  furnaces,  when  finished,  with  a  very 
moderate  fire  for  five  or  six  days,  day  and  night.  Upon  a  slow, 
gradual  drying,  the  durability  of  the  arch  depends.  The  furnace 
must  be  nearly  red  hot  before  the  first  charge  of  ore  is  introduced. 


In  building  a  roasting  furnace  where  the  locality   is   favorable   to 


ROASTING   FURNACES. 


69 


such  an  arrangement,  it  is  a  great  advantage  to  lace  the  furnace  so 
that  the  floor  on  which  the  roaster  stands  should  be  10  or  15  feet 
higher  than  the  cooling  floor,  wherefrom  the  ore  can  be  carried  to  the 
amalgamating  pans  or  to  the  leaching  tubs,  as  represented  by  the 
outline  sketch,  Fig.  6. 

Fig.  6. 


A  Double  Roasting  Furnace  is  represented  in  Fig.  7,  in  longitudi- 
nal cross  section.  The  lower  hearth,  a,  is  nine  feet  long  and  ten 
feet  wide.  The  roof  in  the  center  is  28  inches,  and  at  the  flue  and 


70  BOASTING   FURNACES. 

bridge  fourteen  inches  above  the  hearth.  The  fire-place,  r,  is  twenty 
inches  wide,  eight  feet  long,  and  twenty  inches  from  the  roof.  The 
flue,  b,  ascends  to  the  upper  hearth,  c',  the  working  door,  o,  is  on  the 
back  side.  In  case  there  should  be  required  more  heat  than  is  obtain- 
able from  the  lower  hearth,  there  is  an  auxiliary  fire-place,  r'.  The 
flame  goes  through  the  flue,  b',  into  dust  chambers.  These  chambers 
have  cross  partitions  lengthways,  by  which  the  draught  is  forced  to 
take  a  longer  way  before  it  enters  the  chimney.  From  the  upper 
hearth  the  ore  is  drawn  through  the  flue,  b,  to  the  lower  hearth;  e,  e, 
are  canals  for  the  escape  of  moisture,  and  e  e'  for  the  tie-rods. 
The  two  hearths  can  be  used  separately  if  needed.  This  furnace, 
although  somewhat  inconvenient  for  the  roasters,  has  the  advantage 
of  taking  up  less  space,  and  as  the  ore  drops  from  the  drying  hearth 
on  the*  second,  and  from  there  on  the  first  hearth,  it  effects  a  good 
mixing  of  the  not  uniformly  heated  ore.  The  auxiliary  fire  is  useful 
for  silver  ores,  which  seldom  contain  so  much  sulphur  as  to  burn  long 
enough  without  the  help  of  a  nearer  fire  than  that  from  the  lower 
hearth.  In  working  concentrated  sulphurets,  the  upper  fire  can  be 
dispensed  with.  Deetken,  who  has  had  long  experience  in  roasting 
sulphurets,  considers  this  kind  of  furnaces  superior  to  the  long  ones 
and  to  all  others  he  ever  used.  The  arch  in  the  middle  is  only  eight 
inches  thick,  but,  being  made  level,  the  thickness  increases  towards 
the  sides.  A  few  days  after  starting  up,  the  whole  roof  is  sufficiently 
heated  to  keep  the  entire  mass  of  the  sulphurets  red  hot  throughout, 
and  the  fuel  is  better  utilized;  one  ton  always  being  drawn  from  the 
upper  hearth,  when  the  preceding  charge  is  finished.  The  charge  on 
the  lower  hearth  requires,  generally,  from  eight  to  ten  hours  before 
the  roasting  is  completed.  In  constructing  a  double  furnace,  in  two 
stores,  it  is  essential  to  use  a  number  of  iron  ties  across  the  width  of 
the  furnace  between  the  lower  and  upper  hearth;  otherwise,  on  account 
of  the  weight  of  the  upper  charged  hearth,  the  side  wall  would  inva- 
riably give  way  in  the  course  of  a  while.  Jf  used  for  the  roasting  of 


Fig 


Fig.  9 


72  ROASTING  FURNACES. 

gold  sulphurets,  or  such  ores  as  do  not  require  the  auxiliary  fire^  the 
working  doors  can  be  placed  at  m  and  n,  and,  at  m,  a  hoe  can  be  ad- 
vantageously used  in  drawing  the  charge  to  the  lower  hearth. 

Long  Roasting  Furnaces.  This  kind  of  furnaces,  as  represented  by 
Fig.  8,  pg.  71,  by  a  vertical  section,  and  Fig.  9,  by  the  ground  plan,  gives 
much  satisfaction,  as  there  is  not  only  a  great  saving  of  fuel  effected* 
but  also  a  greater  quantity  of  ore  can  be  roasted  than  with  a  single 
furnace.  It  is  a  modification  of  the  double  furnace,  and  seems  to  be 
more  convenient  for  the  roasters.  There  are  two  men  employed  at  a 
time,  there  being  one  ton  and  a  half  to  two  tons  in  the  furnace.  The 
hearths  are  either  arranged  horizontally,  as  the  drawings  show,  or 
only  the  first  one  is  level,  the  other  two  are  inclined ;  this  facilitates 
the  shifting  of  the  ore.  Each  hearth  is  ten  feet  long  and  twelve  feet 
wide.  After  the  first  hearth  there  is  a  step  of  four  to  six  inches, 
partly  to  divide  the  first  from  the  others,  but  principally  to  effect  a 
better  mixing  of  the  ore.  The  ore  is  fed  on  the  last  hearth, 
through  the  sheet  iron  funnel,  spread  equally  on  the  hearth,  and, 
according  to  its  dampness  or  the  quantity  of  sulphurets  contained, 
stirred  more  or  less  from  one  and  a  half  to  two  hours.  As  it  is  not 
only  inconvenient,  but  impossible,  to  have  a  good  stirring  effected  at 
a  distance  of  twelve  feet,  which  requires  long  and  heavy  tools,  there 
are  for  this  reason  working  doors  on  both  sides  of  the  furnace.  The 
roaster  uses  hoes  or  rakes  eight  feet  long,  made  partly  of  gas  pipe, 
which  are  light  and  handy.  The  working  doors  are  thirty  inches 
wide.  They  must  all  be  kept  closed  except  when  the  ore  is  being 
raked,  and  then  it  is  very  proper  to  have  half  of  the  door  closed  (with 
a  piece  of  sheet  iron).  Sufficient  air  comes  in  at  the  working  door  of 
the  first  hearth. 

After  one  and  a  half  to  two  hours  the  ore  is  removed  to  the 
middle  hearth  and  spread  equally  over,  the  whole  surface.  A  new 
-charge  is  introduced  in  place  of  the  former.  There  is  a  higher  heat 


ROASTING   FURNACES.  73 

on  the  middle  hearth  than  on  the  last  one.  The  treatment  of  the  ore 
is  here  the  same  as  before,  being  raked  from  time  to  time.  After 
a  lapse  of  one  and  a  half  to  two  hours  the  ore  is  moved  again  to 
the  first  hearth,  in  the  same  way  as  before.  The  ore  is  now  exposed 
to  a  light  red  heat,  by  which  the  chlorination  or  oxidation  must  be 
finished  in  the  same  time  as  on  the  other  hearth.  It  is  necessary  to 
change  here  the  ore  from  the  bridge  toward  the  flue,  and  reverse  once 
during  the  roasting.  When  the  operation  is  finished,  the  roasted  ore 
is  drawn  into  iron  cars  below  the  furnace,  through  the  opening  in  the 
floor.  When  all  the  ore  has  been  removed,  the  charge  on  the  second 
hearth  is  transferred  to  the  first,  from  the  third  to  the  second,  and 
from  the  funnel  to  the  third  hearth,  and  the 'process  continued  as  before, 
so  that  a  thousand  pounds  are  drawn  out  every  one  and  a  half  to 
two  hours. 

The  bridge  is  fourteen  inches  high.  For  the  purpose  of  admit- 
ting air  or  steam,  a  canal  can  be  made  in  it.  The  fire-place  is 
eighteen  inches  wide  and  eight  to  nine  feet  long,  and  fifteen  inches 
below  the  top  of  the  bridge.  The  ash-pit  is  made  according  to 
what  seems  more  convenient,  or  as  represented  either  in  Fig.  8  or  in  Fig. 
9.  A  deep  ash-pit  is  more  favorable  for  the  preservation  of  the 
grates,  as  they  are  less  heated.  Each  door  is  provided  with  an  iron 
roller.  A  furnace  of  a  similar  description  was  in  operation  in  La 
Dura  (Mexico),  roasting  refractory  silver  ores  for  the  chlorination 
process. 

A  furnace  sixty  feet  in  length,  with  six  hearths,  as  built  by  Mr. 
Graff  at  the  San  Marcial,  has  the  advantage  of  being  capable  of  roast- 
ing from  eight  and  one-half  to  twelve  tons  of  ore  in  twenty-four  hours, 
discharging  every  hour  from  eight  hundred  to  one  thousand  pounds, 
according  to  the  charge.  *In  case  ore  is  subjected  to  roasting  which 
has  not  enough  sulphur  to  create  the  required  heat  in  burning,  an 
additional  smaller  fire-place  must  be  attached  on  oiie  side,  so  as  to 
bring  the  flame  into  the  fourth  hearth. 


74  ROASTING  FURNACES. 

Muffle  Furnaces. 

A  muffle  furnace,  as  the  name  indicates,  is  a  furnace  constructed  of 
clay  and  cast  iron  in  such  a  way  as  to  prevent  the  flame  from  coming 
Inside  of  it.  The  fuel  heats  the  mantle  or  muffle  from  the  outside, 
so  that  the  ore  is  not  heated  directly  by  the  burning  fuel,  but  by  the 
glowing  muffle.  The  muffle  furnaces  require,  therefore,  more  fuel 
to  obtain  a  certain  degree  of  heat  than  ordinary  reverberatory  fur- 
naces, where  the  flame  comes  into  contact  directly  with  the  ore.  The 
Use  of  this  furnace  is  limited,  and  applicable  in  cases  where  the  air  or 
the  gases  of  the  burning  fuel  are  injurious,  or  where  volatile  sub- 
stances from  the  ore  should  be  condensed ;  as,  for  instance,  sulphur, 
zinc,  arsenic,  etc.  For  roasting  silver  ores,  these  furnaces  are  not  in 
use,  but  they  were  tried  in  California  in  different  ways;  also,  for 
desulphurization,  adding  charcoal  to  the  pulverized  ore.  The  experi- 
ments, however,  were  not  successful,  as  could  have  been  anticipated. 

Mechanical  Furnaces  Fed  by  Charges. 

There  is  a  great  variety  of  mechanical  furnaces  whereby  the  costly 
fetirring  by  hands  is  replaced  by  mechanical  contrivances.  On  account 
of  the  very  different  styles  of  arrangement,  the  description  of  the 
treatment  of  the  furnaces  fed  by  charges,  as  well  as  those  fed  contin- 
uously, will  be  found,  together  with  the  description  of  the  furnace 
itself,  unless  they  agree  in  the  main  points,  like  the  revolving  cylin- 
der furnaces.  No  mechanical  furnace  can  be  governed  in  every  part 
of  the  roasting  process  with  the  same  facility  and  precision  as  is  pos- 
sible in  a  reverberatory  furnace  with  manual  labor ;  but  in  the  latter 
ease  the  great  difficulty  in  finding  good  reliable  roasters,  and  the 
heavy  expenses  connected  therewith,  make  a  mechanical  substitution 
very  desirable.  Amongst  this  class  of  furnaces  are  the  revolving 
cylinders,  through  which  the  flame  passes  the  most  successfully.  The 
fuel  is  here  better  utilized  than  in  a  reverberatory  furnace  ;  but,  being 


ROASTING   FURNACES.  75 

twelve  to  fifteen  feet  long,  the  ore  is  more  exposed  to  the  heat  at  the 
fire-place  than  on  the  opposite  side.  To  avoid  this,  Brickner  uses  a 
diaphragm,  which  does  good  service  in  renewing  the  surface  of  the  ore; 
but,  to  bring  the  ore  from  the  flue  to  the  fire-place  and  back,  it  does 
not  quite  give  entire  satisfaction  in  this  respect.  M.  B.  Dodge  made 
recently  another  arrangement  with  projecting  bricks,  the  result  of 
which  is  not  yet  known.  A  cylinder  of  a  large  diameter  will  heat 
the  ore  more  uniformly  than  of  a  small  one,  both  having  the  same 
length.  Besides  the  cylinders  there  are  furnaces,  the  hearth  of  which 
revolves,  having  a  circular  shape  and  stationary  stirrers.  Other  fur- 
naces have 'a  stationary  round  hearth  and  revolving  stirrers,  etc. 

Bruckner's  Revolving  Furnace.  This  furnace  consists  of  a  cylinder 
of  boiler  iron,  lined  with  fire-bricks  as  tight  as  possible.  This  cylin- 
der revolves  between  a  fire-box  and  the  flue  on  friction  wheels.  The 
flame  passes  from  the  fire-box  direct  into  the  cylinder,  and  from  there, 
with  all  the  gases  that  evolve  during  the  roasting,  into  the  dust  cham- 
bers. Inside  of  the  cylinder  is  the  diaphragm,  made  of  cast-iron  pipes. 
It  is  set  at  an  angle  of  about  fifteen  degrees  to  the  axis.  It  has  a 
diagonal  position  extending  through  the  whole  length  of  the  furnace, 
and  is  intended  to  move  the  ore  from  one  end  to  the  other  in  order  to 
expose  the  cooler  ore  from  the  flue  to  the  stronger  heat  at  the  fire-box. 
In  this  way  the  stirring  is  performed  by  aid  of  the  diaphragm  and  the 
revolution  of  the  cylinder.  The  great  drawback  of  unskilled  or  in- 
different roasters  is  hereby  avoided.  What  the  roaster  has  to  perform 
is  so  simple  that  any  person  of  common  sense  can  learn  the  treatment, 
after  having  been  once  regulated,  in  a  few  days.  The  expenses  for 
labor  and  fuel  are  considerably  less  than  with  reverberatory  furnaces; 
also,  the  time  of  roasting  is  generally  shorter.  If  properly  arranged, 
one  man  can  attend  two  furnaces — that  is,  to  fire  up,  charge  and  dis- 
charge. One  charge  is  from  one  and  a  half  to  two  tons  of  silver  ore, 
and,  according  to  the  ore,  from  five  to  six  per  cent,  of  salt,  which  is 


ROASTING    FURNACES.  77 

charged  together  with  the  ore,  or  towards  the  end  of  roasting.  The 
salt  should  be  ground  fine  and  dry;  if  damp,  it  will  form  small  balls, 
which  remain  so  during  the  whole  process.  There  is  a  manhole  in  the 
cylinder,  through  which  the  charge  is  introduced,  and  the  same  hole 
can  serve  also  for  the  discharge,  or  to  get  the  charge  out  quicker ;  in 
others  there  are  also  two  manholes  in  place  of  one,  one  opposite  the 
other.  When  the  ore  is  charged,  the  door  is  closed  and  the  cylinder 
allowed  to  revolve  at  the  rate  of  one  or  two  revolutions  per  minute, 
and  the  roaster  can  concentrate  his  attention  to  the  one  and  principal 
task,  viz  :  to  keep  up  the  right  temperature.  He  can  judge  of  the 
heat  partly  through  an  opening  that  is  made  expressly  for  this  pur- 
pose opposite  the  flue,  through  which  he  should  be  able  to  take  out  a 
sample  with  a  long  handled  scoop,  and  he  can  also  see  through  the  iron 
pipes  of  the  diaphragm  from  outside,  which  pipes  appear  more  or  less 
red  hot,  according  to  the  heat  inside  of  the  cylinder.  Like  in  a  re- 
verberatory,  so  also  in  Bruckner's,  very  little  fuel  is  used  as  long  as 
there  are  sulphurets  enough  in  the  ore  to  keep  up  the  heat  by  burn- 
ing of  the  sulphur.  The  fire  must  be  regulated  accordingly  and  must 
be  increased  as  soon  as  the  temperature  commences  to  sink.  By  de- 
grees the  heat  is  raised  to  a  greater  intensity.  When  the  reaction 
between  salt  and  sulphates  begins,  the  ore  assumes  a  woolly  consist- 
ency, increasing  in  volume  considerably.  The  disagreeable  smell  of 
sulphurous  acid  disappears  and  chlorine  gas  is  evolved  in  its  place ; 
the  temperature  is  now  increased  to  a  bright  red  heat  for  an  hour, 
and  the  chlorination  is  considered  finished.  The  motion  of  the  cylin- 
der is  stopped  for  a  moment,  the  manhole  door  opened,  and  the  revo- 
lution of  the  furnace  started  with  an  increased  speed  of  from  5  or  6 
revolutions  per  minute. 

In  Colorado,  where  Bruckner's  furnaces  were  successfully  intro- 
duced in  1867,  for  roasting  of  silver  ores  and  gold  sulphurets,  the 
ore  is  made  to  drop  through  a  grate  into  a  screw  conveyor,  and  con- 
veyed in  an  iron  trough  to  the  required  place  for  further  treatment. 


78  ROASTING    FURNACES. 

This  is  a  very  convenient  way  to  move  the  hot  ore  away  from  the 
furnace,  and  less  expensive  than  if  performed  by  hand  labor.  In 
order  to  obtain  satisfactory  results,  the  furnace  must  revolve  slowly, 
about  one  revolution  in  one  or  two  minutes. 

Modified  Brucker's  Furnace.  The  difference  between  this 
modified  cylinder  furnace  and  Bruckner's,  consists  principally  in 
the  omission  of  the  diaphragm,  and  is  thereby  reduced  to  a  simple 
cylinder  furnace.  A  large  sized  cylinder  was  built  in  the  Pacific 
Foundry,  San  Francisco,  capable  of  holding  seven  tons  per  charge. 

In  treating  rebellious  ores,  this  furnace  gives  very  satisfactory  re- 
sults. The  speed  of  revolving  is  best  ascertained  by  experiments, 
trying  first  with  one  revolution  per  minute,  then  one  in  two  minutes, 
and  one  in  three.  In  roasting  ore,  the  treatment  in  this  furnace 
does  not  differ  from  the  preceding. 

B.  Dodge's  Revolving  Furnace.  This  furnace  differs  from  the 
former  in  shape,  being  six  sided,  in  stead  of  round.  The  furnace 
is  seven  feet  in  diameter  and  sixteen  feet  long,  calculated  for  a 
charge  of  seven  tons.  The  gearing  is  so  arranged  as  to  alter 
the  speed  without  altering  the  speed  of  the  driving  engine;  the 
change  of  speed  is  necessary  for  the  discharge,  when  the  roasting  is 
finished.  The  revolving  of  the  furnace  is  effected  by  peculiar 
gearing.  The  bands  encircling  the  furnace  rest  on  a  pair  of 
grooved  rollers.  The  shafts  of  one  pair  of  these  rollers  are  extended 
past  the  bearings,  and  on  their  ends  are  pinions,  between  which  is 
placed  a  gear  wheel.  On  the  opposite  end  of  the  shaft,  carrying  this 
gear,  is  a  crown  wheel,  and  this  being  riveted,  the  center  gear  rotates 
the  pinions  both  in  the  same  direction,  thus  moving  the  rollers  in  the 
same  direction,  and  rotating  the  cylinder,  applying  the  power  on  both 
sides. 

The  six-sided  cylinder  is  lined  with  bricks,  forming  also  peculiar 


ROASTING   FURNACES.  79 

projections,  calculated  to  move  the  ore  from  one  end  to  the  other. 
The  charging  is  effected  through  two  doors;  and  there  are  two  other 
doors  opposite,  so  that  when  all  four  are  open  at  an  increased  speed, 
the  discharge  of  the  seven  tons  takes  but  little  time.  The  doors  are 
so  arranged  that  they  can  be  opened  or  shut  with  the  greatest  ease. 
The  advantage  of  the  six-sided  shape  of  the  cylinder  is  said  to  be  in 
the  fact  that,  as  the  furnace  revolves,  the  ore,  resting  on  the  rising 
side,  remains  stationary,  being  held  there  by  the  corner,  formed  by 
the  juntion  of  the  sides.  At  a  certain  point,  however,  the  whole  mass 
on  that  side  drops  down,  and  is  completely  turned.  The  outer  surface 
of  this  body  then  remains  in  contact  with  the  heat  and  flame  during  the 
time  the  side  which  is  carrying  it  is  raised  up  as  the  furnace  rotates, 
when  another  sudden  fall  of  ore  takes  place,  and  the  operation  is 
repeated.  This  of  course  can  take  place  only  after  the  ore  assumes  a 
woolly  or  spongy  condition.  Some  other  mechanical  furnaces  of  less 
importance,  are  those  of  a  circular  hearth.  Of  this  kind  there  is  the 

Revolving  Hearth  Furnace.  The  shape  of  this  furnace  is  cir- 
cular. There  is  an  iron  frame  of  from  ten  to  twelve  feet  diame- 
ter, with  sides  fourteen  inches  high.  The  whole  is  lined  with 
brick,  the  bottom  four  inches  thick.  The  discharge  opening 
is  on  the  bottom,  extending  from  the  periphery  toward  the 
center,  and  is  four  inches  wide  and  three  feet  three  inches 
long.  This  opening  is  shut  by  an  iron  door,  hung  on  hinges.  It  is 
not  necessary  to  fill  this  space  with  brick,  which  would  interfere  with 
the  easy  opening;  but  the  space,  after  the  discharge  of  the  ore,  must 
be  filled  up  with  roasted  ore,  of  which  enough  is  always  left  in  the 
furnace.  The  bottom  is  fixed  to  an  upright  shaft,  four  inches  in 
diameter,  provided  with  a  spurwheel  at  the  lower  end  to  impart  the 
motion.  This  ten  or  twelve-foot  bottom  is  surrounded  by  a  substan- 
tial ring  wall,  as  close  to  the  periphery  of  the  bottom  as  possible. 
The  bottom  is  then  arched  over  with  bricks,  leaving  the  doors  through 


80  ROASTING   FURNACES. 

which,  the  new  shoes  are  introduced  when  the  old  ones  wear  out. 
There  is  also  a  cast  iron  pipe  through  the  center  of  the  furnace,  on 
which  the  shoes  are  fastened  and  so  arranged  that  one  set  plows  the 
ore  against  the  center,  the  other  set  toward  the  periphery.  The  pipe 
is  hollow  and  cooled  by  a  continual  stream  of  water.  There  is  also 
a  hole  four  to  five  inches  square  in  the  arch,  in  connection  with  a 
funnel,  through  which  the  ore  is  charged  into  the  furnace.  The 
distance  from  the  bottom  to  the  center  of  the  arch  is  thirty-one 
inches.  The  arch  is  connected  on  one  side  with  the  fire-place,  six  or 
seven  feet  long  and  eighteen  or  twenty  inches  wide,  and  about  ten 
inches  below  the  rim  of  the  revolving  hearth  are  the  grates.  On  the 
opposite  side  is  the  connection  with  the  flue. 

Such  furnaces  have  the  advantage  that  they  carry  the  ore  in  a 
circle,  so  that  each  part  is  equally  exposed  to  the  heat  near  the  bridge 
and  to  the  cooler  region  near  the  flue.  While  revolving,  the  funnel 
is  opened  and  the  ore  falls  on  the  moving  bottom,  being  spread  in 
passing  under  the  stationary  stirrers,  which  are  of  a  plow  shape.  The 
roasting  takes  about  the  same  time  as  in  an  ordinary  furnace,  but 
requires  less  fuel,  as  the  furnaces  are  cooled  down  by  air,  which  enters 
the  common  reverberatory  furnace  through  the  working  door.  It  is 
important  to  have  the  horizontal  shaft  provided  with  two  driving 
wheels  of  diflent  size,  so  that  about  one  to  two  revolutions  per 
mieute  can  be  obtained  while  roasting,  and  from  six  to  eight  revolu- 
tians  while  discharging.  After  the  roasting  is  finished,  the  discharge 
door  on  the  bottom  can  be  opened,  while  the  hearth  revolves  slowly. 
In  this  furnace  it  is  an  easy  matter  to  arrange  the  plows  in  such  a 
way  that  they  could  be  moved  every  second  or  third  day  toward  the 
periphery  as  much  as  they  wear  off.  In  this  way  the  side  of  the 
hearth  can  be  kept  always  clear  from  accumulation  of  the  ore  crust. 

A  similar  furnace  is  Burton's  Revolving  Furnace.  The  hearth  has 
a  low  conical  shape,  the  highest  point  being  the  center.  Above  this 


ROASTING    FURNACES.  81 

is  the  charging  hole  in  the  roof.  The  hearth  is  twelve  feet  in  diame- 
ter, and  takes  one  ton  of  tin  ore  at  a  charge.  There  is  a  cast-iron  rake 
with  three-inch  long  prismatic  teeth,  which  are  dovetailed  and  so  con- 
structed as  to  be  easily  replaced.  The  ore  comes  through  a  funnel  in 
the  center  of  the  revolving  hearth,  and  is  spread  by  the  stationary 
rake,  the  position  of  which  is  not  radial  but  oblique.  The  hearth  is 
fixed  to  a  solid  vertical  shaft  with  gearing,  by  which  a  slow  rotating 
motion  is  imparted  to  the  hearth,  so  that  only  one  revolution  is  made 
in  forty  minutes. 

Parke's  Roasting  Furnace,  with  movable  stirrers.  This  is  a  double 
furnace,  one  hearth  above  the  other,  with  a  common  vertical  shaft  to 
which  the  stirrers  are  fastened.  The  hearth  is  twelve  feet  in  diame- 
ter and  rests  on  an  arch,  beneath  which  the  rotating  motion  is  trans- 
ferred to  the  shaft  by  means  of  gearing.  On  one  side  of  the  lower 
hearth  is  the  fire-place,  whence  the  flame  draws  over  the  bridge  into 
the  furnace. 

Opposite  the  bridge  is  an  opening  one  foot  wide  and  four  feet  long, 
through  which  the  flame  ascends  to  the  upper  hearth.  Both  of  the 
hearths  have  two  working  openings,  which  are  closed  by  cast-iron  doors. 
From  the  upper  hearth  the  flame  draws  through  a  flue  into  the  chim- 
ney. The  shaft  goes  through  both  hearths  and  the  roof.  There  are 
two  massive  arms  in  both  furnaces,  with  curbed  spikes  attached  for 
the  purpose  of  stirring  the  ore.  In  order  to  keep  the  shaft  cool,  it  is 
hollow,  and  a  few  holes  above  the  gear  permit  the  cold  air  to  draw 
through  the  shaft,  whereby  a  constant  cooling  is  effected.  The  upper 
end  of  the  shaft  runs  in  a  cast-iron  cross,  fixed  on  the  roof  of  the 
furnace. 

After  the  ore  on  the  lower  hearth   is   drawn  out  through  the  dis 
charge  hole  at  the  bottom,  the  ore  on  the  upper  hearth,   already  de- 
sulphurized to  a  great  extent,  is  raked  toward   a  similar  discharge 
hole,  and  then  transferred  to  the  lower  department.     The  raw  ore  13 


82  ROASTING   FURNACES. 

charged  through  the  roof  into  the  upper  part.  By  means  of  hoes  the 
ore  is  spread  on  both  hearths,  before  the  shaft  is  allowed  to  revolve 
again. 

Mechanical   Furnaces  with   Continuous  Feeding. 

This  class  of  furnaces  has  a  great  advantage  over  the  preceding 
ones,  inasmuch  as  there  is  still  less  manual  labor  connected  with  the 
whole  operation  of  roasting,  charging  and  discharging.  From  the 
crushing  machinery  the  ore  pulp  is  conveyed  by  screws,  and,  if  neces- 
sary, by  elevators  to  the  furnace,  and  the  roasted  ore  is  discharged 
mechanically  in  the  same  proportion  as  it  is  fed.  The  arrangement 
of  these  furnaces  differs  in  construction  very  much,  as  will  be  seen 
from  the  following  description  : 

The  Howell  Improved  White  Furnace.*  This  furnace  consists 
of  a  tellescope-shaped  iron  cylinder,  as  represented  in  figure 
10.  The  wider  part  of  the  furnace,  where  the  flame  enters,  is 
lined  with  fire-bricks;  the  other  narrower  part  of  the  cylinder  is 
not  lined  at  all,  but  stands  the  less  intense  heat  very*  well;  it  re- 
ceives during  the  operation  a  kind  of  ore-coating,  which  protects 
the  iron.  The  cylinder  has  a  slightly  inclined  position,  and  is  of  a 
very  simple  construction;  the  revolving  arrangement  is  much  improved 
and  durable.  The  usual  size  is  40  inches  in  diameter  and  24  feet 
long,  capable  of  roasting  10  to  20  tons  of  ore.  The  largest  cylinder 
is  sixty  inches  by  twenty-four  feet,  and  calculated  for  thirty-five  to 
forty-five  tons.  The  forty  inch  furnace  requires  two  horse  power, 
and  weighs  ten  tons.  It  is  made  in  sections  for  transportation  by 
wagons  or  mule  backs. 

The  main  fire  is  at  the  lower  or  discharge  end  of  furnace.  The 
flame  passes  over  the  ore  receiving  pit,  directly  into  the  cylinder,  and 

*This  furnace  is  patented  and  made  in  the  Pacific  Foundry,  San  Francisco. 


ROASTING   FURNACES.  83 

through  it  towards  the  flue ;  the  ore  on  the  contrary  is  fed  on  the 
opposite  side,  the  upper  end,  and  progresses  against  the  flame  by 
degrees  into  the  higher  heat.  The  ore  is  regularly  fed  directly  from 
the  battery  or  some  other  crushing  apparatus,  by  means  of  a  screw 
conveyor,  either  with  salt  from  the  battery,  or  the  salt  can  be  fed 
mechanically  outside  the  battery  by  a  salt-feeder,  which  can  be  easily 
regulated  to  feed  the  required  percentage.  In  the  lining  of  the 
cylinder  there  are  brick  projections  about  two  feet  apart,  and  in  the 
upper  part  which  is  not  lined  with  bricks,  there  are  flanges  to  prevent 
the  sliding  of  the  ore,  and  to  carry  it  up  by  the  revolving  furnace  till  it 
drops  through  the  red  hot  air  and  gases,  to  be  lifted  again.  This 
action  is  constantly  repeated;  the  ore  at  each  drop  comes  nearer  to 
the  discharge  end  and  into  a  higher  temperature,  till  it  drops  out  into 
the  ore-receiving  pit,  well  roasted  and  chloridized  to  from  85  to  05 
per  cent,  of  the  assay  value. 

By  the  constant  showering  of  the  ore  and  the  draught,  the  dust  of 
the  ore  becomes  separated  from  the  sandy  part,  and  is  carried  out  of 
the  furnace  towards  the  flue,  before  it  can  reach  the  necessary 
temperature  to  be  thoroughly  chloridized.  For  this  reason  an  auxili- 
ary fire  is  placed  at  the  upper  end  of  the  furnace,  the  heat  from  which 
comes  in  contact  with  this  fine  dust,  and  passes  with  it  under  an 
arch,  and  through  the  flue  to  the  dust  chambers.  In  this  way  the 
dust  is  as  well  roasted  as  the  ore  in  the  ore-pit,  and  generally  shows 
even  a  higher  chlorination. 

The  Howell  improved  White  furnace  of  40  inches  in  diameter  at 
the  lower  end,  and  30  inches  at  the  upper,  and  24  feet  long,  is 
sufficient  for  any  ten-stamp  mill,  although  with  some  ores,  also  for 
15  stamps.  For  twenty-stamp  mill,  the  furnace  must  be  proportion- 
ately larger.  To  increase  the  working  capacity  of  these  furnaces,  it 
is  only  necessary  to  add  to  the  diameter,  as  24  feet  is  long  enough  for 
the  largest  size. 


84  ROASTING   FURNACES. 

The  proportion  of  the  ore  deposited  in  the  dust  chambers  compared 
with  that  in  the  ore  pit,  ranges  from  30  to  50  per  cent.  This  division 
of  the  ore  in  two  different  places  should  not  be  considered  a  very 
great  inconvenience  if  only  a  high  chlorination  is  obtained  on  both 
sides.  It  takes  from  one  to  two-tenths  of  a  cord  of  wood  to  roast 
one  ton  of  ore,  and  from  three  to  five  per  cent,  of  salt,  which,  how- 
ever, must  be  increased  with  rich  ore,  as  is  the  case  with  all  furnaces. 
The  cylinder  revolves  from  five  to  six  times  per  minute,  sometimes 
less;  it  depends  on  the  quality  of  ore.  For  plumbiferous  ores,  five 
revolutions  per  minute  would  probably  be  too  much. 

Treating  plumbiferous  ores,  the  cleaning  of  the  furnace  once  a  week 
should  not  be  neglected,  otherwise  the  coating  would  increase,  and 
finally  stop  the  roasting.  These  furnaces  are  already  introduced  in  a 
great  number  of  mills  in  several  States  and  in  Mexico,  being  in 
successful  operation,  the  chlorination  of  the  silver  ranging  between 
80  and  96  per  cent. 

O'ffarra's  Mechanical  Furnace.  This  furnace  was  first  tried  in 
1862  or  1863,  in  Dayton,  Nev.;  and  later,  three  of  them  were  built 
in  Flint,  Idaho  Ty.  The  main  feature  of  this  furnace-is  the  endless 
chain  to  which  two  oval  rings  are  attached,  the  rings  being  as  wide  as 
the  cross  section  of  the  hearth.  To  these  rings  are  fastened  the 
ploughs  or  shoes  by  which  the  ore  is  gradually  pushed  forward.  The 
construction  of  O'Harra's  furnace  that  was  built  in  Flint,  is  shown  by 
an  outline  drawing,  as  represented  in  Fig.  11.  The  hearth,  A,  is  104 
feet  long  and  nearly  five  feet  wide.  Eighty  feet  of  this  hearth  are 
crossed  by  an  arch,  £,  twelve  inches  high,  and  connected  with  three 
fire-places — two,  c  and  d,  on  one  side,  and  one  between  c  and  d  on  the 
other,  a  is  the  feeding  hearth,  provided  with  ore  continuously 
from  the  batteries.  The  motion  of  the  ore  is  effected  by  an  end- 
less chain,  g,  passing  over  two  chain  wheels,  one  at  each  end.  To 
this  chain  two  oblong  flat  rings,  h,  are  attached,  each  provided  with 
eight  shovels  or  plows,  so  arranged  that  while  one  of  the  rings  shoves 


ROASTING   FURNACES. 


85 


the  ore  toward  the  center  line,  the  other  pushes  it  back 
again  toward  the  sides  every  three  or  four  minutes,  (or 
in  shorter  intervals  if  more  ore  is  charged)  The  ore  not 
only  changes  its  place  to  the  right  and  left,  but  it  also 
moves  forward  by  degrees,  so  that  in  the  course  of  six 
hours  from  the  beginning,  it  commences  to  be  dis- 
charged at  ft  passing  eighteen  feet  over  the  cooling 
hearth,  e.  On  both  ends  of  the  furnace  are  iron 
doors  hung  on  hinges,  which  are  opened  by  the  rings. 
After  several  months  of  operation  the  hearth  or  bot- 
tom appeared  in  good  condition. 

The  five  batteries,  five  stamps  each,  have  on  both 
long  sides  endless  screws,  by  which  the  crushed  ore  is 
forwarded,  in  proportion  as  it  is  discharged,  to  an  ele- 
vating apparatus.  Being  lifted  about  fifteen  feet,  it  is 
conveyed  again  by  endless  screws  along  the  feeding 
hearths  of  all  three  furnaces,  a',  and  regularly  divided 
and  discharged  on  the  feeding  hearth,  a.  The  ore, 
mixed  with  100  pounds  of  salt  to  each  ton,  is  spread 
on  iron  plates  before  the  batteries,  (heated  by  the  hot 
air  from  the  furnaces,  conveyed  through  the  flue  and 
under  the  plates.)  When  charged  into  the  battery,  the 
bre  is  not  further  handled  till  it  comes  out  of  the  fur- 
naces perfectly  roasted. 


There  is  only  one  obstacle  connected  with  this  and  other  mechanical 
furnaces.  The  shoes  or  shovels,  touching  the  sides  of  the  furnace, 
wear  off  by  degrees,  leaving  a  space  which  is  taken  up  by  the  ore. 
This  part  of  the  ore  along  the  wall,  hardens  and  increases  in  amount 
in  the  furnace  till  new  shoes  are  put  in.  By  these  the  crust  of  one- 
half  to  three-quarters  of  an  inch  thick  is  broken  off  and  carried  out. 

From  the  Eising  Star  ore  these  crusts  contain  nearly  just  as  much 
6 


86  ROASTING   FURNACES. 

chloride  of  silver  as  the  well  roasted  ore;  they  are,  nevertheless, 
disagreeable,  but  some  means  might  be  devised  by  which  this  incon- 
venience could  be  avoided. 

The  ore  from  the  Rising  Star  mine,  at  Flint,  contained  argentiferous 
fahl  ore,  miargyrite,  ruby  silver,  zincblende,  galena,  iron  pyrites  and 
sulphide  of  antimony.  On  an  average  the  ore  paid  between  $90 
and  $100  per  ton,  containing  some  gold.  The  gangue  is  quartz.  It 
was  crushed  through  sieves  with  forty  holes  to  the  inch,  together 
with  5  per  cent,  of  salt.  The  furnace,  Figure  11,  is  charged  continually 
by  machinery  at  one  end,  a.  The  ore  was  moved  by  degrees  forward, 
and  arriving  at  the  first  fire-place,  c,  commenced  to  discharge  sulphur. 
Between  this  fire-place  and  the  second,  which  is  on  the  opposite  side, 
between  c  and  d,  the  chlorination  begins  at  an  increased  heat.  The 
flame  shows  partly  a  blue  color,  originating  from  chloride  of  copper, 
and  white  fumes  are  also  evolved.  Between  the  second  and  the  third 
fire-place,  d,  the  chlorination  is  finished  at  a  light  red  heat.  From 
the  cooling  hearth,  e,  the  roasted  ore  is  continually  discharged  on  the 
dump,  /  It  takes  six  hours  before  the  ore  from  the  feeding  place,  a, 
arrives  at  the  dump.  Although  not  more  than  5  per  cent,  of  salt  is 
added,  the  roasted  ore  contains  about  90  per  cent,  of  the  silver  con- 
verted into  a  chloride.  The  gases,  containing  free  chlorine  and  chlo- 
ride combinations  emitting  chlorine,  being  in  contact  with  the  surface 
of  the  ore  while  passing  over  it  for  a  space  of  eighty  feet,  have  a  chlo- 
ridizing  influence  on  it,  replacing  thus  a  certain  amount  of  salt. 

These  three  furnaces  roasted  twenty  tons  of  ore  in  twenty-four 
hours.  The  expenses  were  as  follows : 

For  wood,  five  cords,  at  $5 $  25  00 

For  four  men  at  the  furnaces,  at  $4 16  00 

For  two  men  bringing  in  wood,  etc..  at  $4 8  00 

For  one  man  as  watch  in  the  night 4  00 

For  blacksmith  work 500    . 

For  2,000  pounds  of  salt,  at  8c 160  00 

Total  expense $218  00 

or  $10.90  per  ton.     The  capacity  of  the  three  furnaces  is  calculated 


ROASTING   FURNACES.  87 

for  more  than  twenty  tons.  Each  one  could  easily  treat  ten  tons  of 
the  Rising  Star  ore  in  twenty-four  hours.  The  roasted  ore  was  treated 
by  amalgamation  in  pans,  applying  the  "leaching  process." 

O'Harra's  furnace  is  now  greatly  improved.  It  is  built  in  two 
stories,  so  that  when  the  chain  comes  out  of  the  lower  hearth  it  turns 
into  that  of  the  upper  story.  The  chain  is  heavy,  and  there  is  no 
trouble  whatever,  either  with  the  chain  or  any  other  part  of  the 
arrangement.  Although  the  chain  in  its  course  through  the  red  hot 
furnace  is  exposed  to  red  heat,  it  nevertheless  does  not  become  so  hot 
as  to  suffer -any  injury  from  itj  there  is  a  wooden  frame  work  on  each 
side  of  the  furnace  over  which  the  chain  and  plows  move  in  the  open 
air,  which  prevents  them  from  getting  too  hot.  The  furnace,  of  the 
latest  construction,  is  eight  feet  wide  and  from  forty  to  a  hundred  feet 
long,  with  four  fires,  two  on  each  side,  directly  opposite  each  other. 
The  first  two  fires,  where  the  ore  comes  in  contact  therewith,  are 
divided,  so  that  one-half  of  the  flame  goes  direct  to  the  lower,  and  the 
other  half  to  the  upper  hearth,  through  an  opening  in  the  arch  of  the 
fire  chamber.  There  is  a  fire-clay  damper  to  regulate  the  flame.  The 
other  two  fires  are  opposite  each  other,  so  that  the  heat  is  uniform 
over  the  whole  hearth.  These  last  two  fires  are  regulated  by  the 
ash-pit  dampers. 

The  endless  chain  has  two  triangular  frames,  with  plow  shoes  on 
each  side.  The  cooling  space  is  built  in  proportion  to  the  length  of 
the  hearth.  A  furnace  of  this  kind,  fifty  feet  long  and  eight  feet 
wide,  can  roast  from  thirty  to  forty  tons  in  twenty-four  hours  with 
the  help  of  only  two  men,  consuming  about  two  cords  and  a  half  of 
wood.  The  chlorination  runs  up  to  90  and  95  per  cent.  One  of 
O'Harra's  furnaces  works  forty  tons  per  day  of  the  old  Ophir  tailings 
(Nevada),  which,  it  is  said,  assay  only  $16.50  per  ton. 

The  working  of  this  furnace  is  not  expensive,  as  one  man  can 
attend  the  roasting  of  forty  tons  in  day  time,  and  one  at  night. 


88  ROASTING  FUBNACES. 

A  remarkable  feature  of  O'Harra's  furnace  is  the  very  small  amount 
of  dust  that  is  carried  off  by  the  draught;  in  fact  there  is  no  other  roast- 
ing furnace  that  could  be  compared  with  O'Harra's  in  this  respect. 
Another  peculiarity  is  the  adaptability  of  this  furnace  for  drying  ore 
in  pieces  the  size  of  a  man's  fist.  One  furnace  dries  40  tons  of 
ore  in  24  hours,  (near  Shasta,  Cal.)  at  a  small  expense. 

The  Stetefeldt  Roasting  Furnace.  The  mechanical  part  of  the  roast- 
ing itself  in  this  furnace,  is  the  simplest  of  all,  and  also  the  shortest. 
The  finely  pulverized  ore,  mixed  with  salt,  is  sifted  continuously  by 
a  mechanical  arrangement  into  a  shaft.  This  shaft  is  about  twenty- 
five  feet  high,  and  heated  by  two  fire-places  provided  with  grates. 
The  ore,  falling  through  the  heated  shaft,  undergoes  chlorination, — a 
process  requiring  only  a  few  seconds.  After  the  roasted  ore  has 
accumulated  on  the  bottom  of  the  shaft  to  the  amount  of  about  1,000 
pounds  it  may  be  drawn  out.  The  amount  of  salt  needed  for  chlori- 
nation, varies  according  to  the  ore;  generally  about  6  per  cent.,  or 
120  pounds  to  the  ton,  is  taken,  or  even  less,  especially  in  treating 
poor  ores,  when  half  of  that  amount  may  be  sufficient  in  most  cases. 
A  furnace  having  a  capacity  of  from  fifteen  to  twenty  tons  in  twenty- 
four  hours,  consumes  from  two  to  three  cords  of  wood.  In  twenty- 
four  hours  there  are  employed:  Two  men  attending  the  feeding 
and  conveying  machinery,  three  firemen,  and  three  men  to  draw  and 
cool  the  roasted  ore.  As  the  latter  three  have  time  enough  to  carry 
the  ore  to  the  pans,  only  half  of  their  time  should  be  charged  to  the 
roasting  expenses.  According  to  these  figures,  the  total  expense  of 
roasting  in  Reno,  for  instance,  was  not  more  than — 

For  labor  of  6£  men  at  $3 $19  50 

For  Wood,  2J  cords  at  $6 15  00 

Salt,  1,800  pounds  at  l£o 27  00 

Total  expense  on  15  tons $61  50 

or  $4.10  per  ton.      From  88  to  97  per  cent,  of  the  silver  contained 
in  the  ore  is  converted  into  a  chloride.     Of  the  dust  in  the  dust- 


ROASTING   FURNACES.  89 

chambers,  the  silver  was  found  in  the  state  of  a  chloride  up  to  96  per 
cent. 

It  is  evident  that  with  an  improper  treatment  of  the  fire,  by  using 
too  much  or  too  little  fuel,  a  less  favorable  result  would  be  obtained. 
In  the  first  place,  if  the  temperature  is  kept  too  high,  a  part  of  the 
chloride  of  silver  is  reduced  to  a  metallic  state,  which,  for  the  purpose 
of  amalgamation,  is  not  so  very  injurious;  but  the  metallic  silver  is  a 
total  loss  with  the  lixiviation  process.  On  the  other  hand,  if  there  is 
not  sufficient  heat,  some  of  the  sulphurets  may  remain  undecomposed. 
In  either  case  the  responsibility  is  with  those  in  charge  of  the  furnace; 
but  there  is  nothing  easier  than  to  keep  up  a  proper  and  uniform 
heat  in  Stetefeldt's  furnace,  there  being  no  other  hand-work  about  it, 
and  all  the  attention  of  the  fireman  being  directed  to  this  single 
point. 

Some  ores  require  more  heat  than  others ;  this  has  to  be  found  out 
experimentally.  Some  ore  gives,  at  a  low  heat,  a  high  percentage  of 
chlorination,  but  the  bullion  result  may  be  very  base ;  giving,  then,  a 
better  heat,  the  bullion  will  improve  a  great  deal  in  fineness  without 
losing  on  the  percentage  of  chlorination.  It  is  essential  to  have  the 
control  of  a  good  draught,  and  in  case  there  should  be  a  defect  in  this 
respect,  the  furnace  must  be  examined,  especially  the  flue,  whether 
an  accumulation  of  ore  dust  does  not  prevent  the  draught  somewhere. 

Generally,  the  bullion  of  the  roastings  in  Stetefeldt's  furnace  is 
finer  than  of  the  same  ore  when  roasted  in  a  reverberatory,  in  the 
usual  way,  because,  to  obtain  a  fine  bullion  from  a  reverberatory,  it 
requires  great  attention  as  well  as  much  longer  time  for  roasting,  in 
order  that  all  base  metals  be  transformed  into  oxides;  while  in  Stete- 
feldt's furnace,  a  larger  proportion  of  the  chloride  of  copper  volatilizes, 
and  all  lead  becomes  a  sulphate,  according  to  an  analysis  of  the  Onta- 
rio ore,  made  by  Stetefeldt. 

A  furnace  of  this  kind  was  built  in  Austin,  Nevada.     The  furnace 


90  ROASTING   FURNACES. 

has  three  important  departments.  1st.  The  roasting  shaft,  twenty- 
five  feet  high  and  five  feet  wide  at  the  bottom,  narrowing  somewhat 
toward  the  top,  to  prevent  the  adherence  of  dust  to  the  wall.  It  is  a 
simple  shaft  of  common  bricks,  built  as  smooth  as  possible.  On  the 
top  of  the  shaft  is  placed  an  iron  feeder,  through  which  a  permanent 
and  uniform  feeding  of  the  pulverized  ore,  already  mixed  with  salt,  is 
effected.  The  ore  falls  on  the  bottom,  and  when  half  a  ton  or  a  ton 
is  accumulated,  it  is  drawn  out  through  the  door.  2d.  The  fire-places. 
There  are  three  gas  generators,  constructed  similarly  to  that  of  the 
copper-refining  furnace  at  Mansfield,  Prussia.  The  cover  is  taken  off 
and  the  charcoal  introduced.  The  cover  is  placed  again  on  its  frame, 
which  contains  sand  in  a  groove,  in  order  to  shut  off  the  draught  en- 
tirely. The  slide  door  is  drawn  out,  and  the  charcoal  falls  on  the 
grate,  through  which  as  much  air  is  admitted  as  is  necessary.  There 
are  also  two  canals  on  each  side  of  the  grate.  Through  these  canals 
is  regulated  the  admission  of  the  air  for  oxidizing  or  burning  the 
carbonic  oxide,  created  above  the  grate.  In  the  flue,  air  and  gas  meet 
together,  and  the  burning  product  heats  the  furnace.  Two  of  these 
generators  heat  the  shaft.  The  flue,  as  well  as  the  generators  above 
the  grates,  are  lined  with  fire-bricks.  The  gas  generators  have  been 
changed  into  chambers  for  wood  fire.  3d.  The  dust  chambers.  With 
the  draught,  the  gases  from  the  shaft,  with  a  part  of  the  fine  ore  dust, 
pass  through  the  vertical  flue,  then  through  the  horizontal  one,  into 
a  series  of  chambers  of  different  sizes.  The  first  four  chambers  are 
smaller  than  the  four  following ;  from  the  last  chamber  the  gases 
draw  into  the  chimney.  The  dust  can  be  removed  from  the  bottom 
of  the  chambers  through  doors.  Almost  all  the  dust  is  regained,  and 
not  in  a  raw  condition,  as  from  dust-chambers  of  reverberatory  fur- 
naces, requiring  re-roasting,  but  perfectly  chloridized,  which  is  prin- 
cipally due  to  the  auxiliary  generator  and  the  longer  contact  with  the 
chlorine  gases. 


ROASTING   FURNACES.  91 

The  furnace  at  Austin  is  calculated  to  roast  from  twenty-five  to 
thirty  tons  of  ore  in  twenty-four  hours,  at  a  cost  of  from  $5  to  $6 
per  ton,  while  the  expenses  in  usual  reverberatory  roasting  furnaces 
at  Austin  amount  to  $12  or  $14  per  ton. 

It  may  be  remarked  that  wherever  the  use  of  the  leaching  process 
appears  admissible  on  silver  ores,  this,  in  connection  with  Stetefeldt's 
roasting,  will  allow  the  most  economical  extraction  of  silver,  even 
from  very  rebellious  ores.  The  baking  of  the  ore  during  the  chlori- 
nation,  in  the  presence  of  lead,  connot  take  place  in  Stetefeldt's  furnace, 
and  it  is  therefore  very  probable  that  a  higher  amount  of  lead  will  be 
less  injurious  than  in  any  other  roasting  process. 

Considering  the  old,  or  rather  the  usual,  theory  deduced  from  the 
roasting  process  in  common  reverberatory  furnaces,  that  sulphates 
must  be  formed  before  the  salt  can  be  decomposed,  and  not  till  then 
will  the  chlorination  begin,  it  would  seem  that  for  these  chemical 
reactions  more  time  is  required  than  a  few  seconds;  but  this  is  not 
the  case.  As  soon  as  ore  and  salt  enter  the  furnace,  each  sulphuret 
particle  ignites  in  the  glowing  atmosphere,  evolving  at  the  same  time 
sulphur,  which,  in  presence  of  the  oxygen  of  the  atmospheric  air, 
coming  undecomposed  through  the  grates,  is  turned  into  sulphurous 
acid  and  the  metal  into  an  oxide,  or  in  part  directly  into  a  chloride. 
The  sulphurous  acid,  in  contact  with  the  ore  particles  and  oxygen,  be- 
comes sulphuric  acid.  The  temperature  is  from  the  start  too  high  to  per- 
mit the  formation  of  sulphates,  so  that  the  sulphuric  acid  turns  its  force 
on  the  red  hot  salt  particles,  setting  the  chlorine  free.  All  these  reac- 
tions are  performed  instantaneously.  Steam,  emanating  from  the  fuel, 
is  also  amongst  the  gases,  consequently  the  creation  of  hydrochloric 
acid  must  ensue.  The  whole  space  in  the  furnace  is  filled  with  glowing 
gases  of  chlorine,  hydrochloric  acid,  sulphurous  and  sulphuric  acid, 
oxygen,  steam,  volatile  base  metal  chlorides,  etc. — all  of  them  acting, 
decomposing  and  composing,  on  the  sulphurets  with  great  vigor.  The 


92  ROASTING  FURNACES. 

chlorine  decomposes  the  sulphurets  directly,  forming  chloride  of 
metals  and  chloride  of  sulphur;  it  attacks  decomposingly  also  oxides 
and  sulphates,  if  present.  The  hydrochloric  acid  performs  the  same 
office.  Also  metallic  silver,  if  it  should  occur  ;n  the  ore,  would  com- 
bine with  the  chlorine.  The  sulphuric  acid,  besides  decomposing  the 
salt,  oxidizes  partly  the  sulphurets,  directly,  etc. 

Considering  now  an  ore  particle  in  a  red  hot  condition  attacked 
simultaneously  by  all  these  gases  while  falling,  the  final  chloridizing 
result  is  inevitable.  The  finer  the  ore  particles  are,  the  more  perfect 
the  chlorination;  but  even  if  some  coarser  parts  (to  a  certain  degree) 
should  reach  the  bottom  not  thoroughly  chloridized,  this  would  be 
finished  in  the  pile,  as  the  chlorination  and  evolution  of  chlorine  gas 
continues  in  the  red  hot  accumulation  on  the  bottom  of  the  furnace, 

Since  1869  there  have  been  erected  twenty-two  Stetefeldt  furnaces 
in  the  United  States,  although  a  great  many  of  these  are  idle  now,  the 
mines  having  failed  to  supply  ore. 

The  latest  built  furnaces  are  greatly  improved.  Fig.  1  shows  the 
construction  of  a  furnace  built  at  the  Ontario  Mill,  Utah. 

a  is  the  shaft  into  which  the  pulverized  ore  is  showered  by  the 
feeding  machine,  placed  on  the  top  of  the  cast-iron  frame,  b.  The 
shaft  is  heated  by  two  fireplaces.  The  ashpits  of  these  are  closed 
by  iron  doors,  having  an  opening,  e,  provided  with  a  slide,  so  that 
more  or  less  air  can  be  admitted  below  the  grate,  and,  consequently, 
more  or  less  heat  generated.  In  order  to  obtain  a  perfect  combustion 
of  the  gases,  leaving  the  firebox  through  the  slit,  t,  an  airslit,  u,  con- 
nected with  the  airchannel,  f,  is  arranged  above  the  arch  of  the  fire- 
box. This  slit  also  supplies  the  air  necessary  for  the  oxidation  of  the 
sulphur  and  the  base  metals.  Another  advantage  of  this  construc- 
tion is  that  the  arches  above  the  firebox  and  firebridge  are  cooled  and 
pevented  from  burning  out.  The  roasted  ore  accumulates  in  the 
hopper,  &,  and  is  discharged  into  an  iron  car  by  pulling  the  damper,  I, 


BOASTING  FURNACES.  93 

which  rests  on  brackets  with  friction  rollers,  m.  n  is  an  observation 
door,  and  also  serves  for  cleaning  the  firebridges,  o  are  doors  to  ad- 
mit tools  in  case  the  roasted  ore  is  sticky  and  adheres  to  the  walls. 
The  gases  and  fine  ore  dust,  which  forms  a  considerable  portion  of  the 
charge,  leave  the  shaft  through  the  flue,  g.  The  doors,  r,  are  pro- 
vided to  clean  this  flue,  which  is  necessary,  with  some  ores,  about 
pnce  a  month,  d  is  an  auxiliary  fireplace,  constructed  in  the  same 
manner  as  the  fireplaces  on  the  shaft,  which  is  provided  to  roast  the 
ore  dust,  escaping  through  the  flue,  g,  in  passing  through  the  chamber, 
h.  p  are  doors  for  observation  and  cleaning.  The  larger  portion  of 
the  roasted  dust  settles  in  the  chamber,  v,  provided  with  discharge 
hoppers,  t,  from  which  the  charge  is  drawn  into  iron  cars  by  moving 
the  dampers,  s.  The  rest  of  the  dust  is  collected  in  a  system  of  dust 
chambers,  q,  connected  with  a  chimney  which  should  rise  from  forty 
to  fifty  feet  above  the  top  of  the  shaft.  At  the  end  of  the  dust  cham- 
bers is  a  damper  by  which  the  draught  of  the  furnace  can  be  regu- 
lated. The  dry  kiln  can  also  be  used  as  a  dust  chamber,  and  the 
waste  heat  of  the  furnace  utilized  for  drying  the  ore  before  crushing 
it.  The  firing  of  the  furnace  is  done  on  one  side,  and  all  discharges 
are  located  on  the  opposite  side. 

The  Feeding  Machine  is  shown  in  Figure  2.  The  cast-iron  frame, 
a,  which  is  placed  on  top  of  the  shaft,  is  provided  with  a  damper,  b, 
which  is  drawn  out  when  the  furnace  is  in  operation,  but  inserted 
when  the  feeding  machine  stops  for  any  length  of  time,  or  if  screens 
have  to  be  replaced,  c  is  a  cast-iron  grate,  to  the  top  of  which  is 
fastened  the  punched  screen,  d.  The  latter  is  made  of  Russia  sheet- 
iron,  or  of  cast-steel  plate,  with  holes  of  one-eighth  to  one-tenth  of  an 
inch  in  diameter.  Above  the  punched  screen  is  placed  a  frame,  e,  to 
the  bottom  of  which  is  fastened  a  coarse  wire  screen,  /,  generally  No. 
3,  made  of  extra  heavy  iron  wire.  The  frame,  e,  rests  upon  friction 
rollers,  g.  The  brackets,  h,  which  hold  the  friction  rollers,  can  be 
raised  or  lowered  by  set  screws,  so  that  the  wire  screen,  /,  can  be 


94  BOASTING   FURNACES. 

brought  more  or  less  close  to  the  punched  screen,  d.  The  brackets,  k, 
carry  an  eccentric  shaft,  I,  connected  with  the  shaft,  m,  from  which 
the  frame,  e,  receives  an  oscillating  motion.  To  the  brackets,  n,  are 
fastened  transverse  stationary  blades,  o,  which  come  nearly  in  contact 
with  the  wire  screen,/,  and  can  be  raised  or  lowered  by  the  nuts, p. 
These  blades  keep  the  pulp  in  place  when  the  frame,  e,  is  in  motion, 
and  also  act  as  distributors  of  the  pulp  over  the  whole  surface  of  the 
screen.  The  hopper,  i,  receives  the  ore  from  an  elevator  which  draws 
its  supply  from  a  hopper  into  which  the  pulverized  ore  is  discharged 
from  the  crushing  machinery.  The  ore  is  generally  pulverized  through 
a  No.  40  screen.  By  means  of  a  set  of  cone  pulleys  the  speed  of  the 
frame,  e,  can  be  changed  from  twenty  to  sixty  strokes  per  minute, 
whereby  the  amount  of  ore  fed  into  the  furnace  is  regulated.  This 
can  also  be  done,  to  some  extent,  by  changing  the  distances  between 
the  punched  screen,  d,  the  wire  screen,/,  and  the  blades,  o. 

The  arrangement^of  the  feeding  and  conveying  machinery  has  been 
lately  much  improved  and  simplified,  so  that  no  heavy  and  large 
building  is  required  on  top  of  the  furnace,  and  the  fireman,  can  easily 
regulate  the  supply  of  ore  to  the  feeding  machine,  and  keep  the  same 
in  running  order. 

Considering  the  enormous  capacity  of  the  Stetefeldt  furnace,  and 
its  durability,  if  well  constructed,  it  is  by  far  the  cheapest  furnace  in 
regard  to  original  cost  of  erection. 

The  largest  sized  furnace,  as  represented  in  the  drawing — the  scale 
of  which  is  1  in.  =  12  ft. — capable  of  roasting  from  50  to  70  tons  of 
ordinary  ores,  and  from  30  to  35  tons  of  very  base  sulphur et  ores  in 
24  hours,  requires  the  following  amount  of  materials,  from  which 
the  cost  of  construction  can  be  easily  calculated  by  any  architect  or 
millwright,  via : 


ROASTING   FURNACES.  95 

1,500  bricks,  for  fire  boxes  and  arches  exposed  to  flame. 
200,000  common  bricks,  of  good  quality,  for  furnace,  large  system  of 
dust  chambers,  chimney,  and  cooling  floor. 

2,500  Bbs.  in  bolts  and  nuts  for  anchoring  furnace  and  dust  cham- 
bers. 

4,500  Bbs.  in  wrought  iron  braces,  flat  iron  for  car-guides,  tools,  etc. 
16,000  Ibs.  in  castings. 

All  the  castings  are  very  plain  and  simple,  the  water-jacket  on  top 
of  furnace,  and  the  water  damper,  having  been  discarded.  Consider- 
able work  is  only  required  on  the  feeding-machine,  feeding-machine 
damper,  and  discharge  damper,  and  some  on  the  fire-doors,  which  will 
be  covered  by  an  additional  charge  of  about  $700  added  to  the  ordi- 
nary price  of  castings. 

The  cost  of  three  iron  discharge  cars  is  $125.  For  furnace  of  15 
to  20  tons  capacity,  without  hopper  discharge,  and  a  less  extensive 
system  of  dust  chambers,  the  amount  of  materials  required  may  'be 
estimated  at  two-thirds  of  the  figures  given  above. 

The  cost  to  erect  a  Stetef  eldt  Furnace  of  largest  size,  at  stated  prices 
for  materials,  freight  and  labor,  is  the  following: 

1,500  fire-bricks,  at  $60  per  M $     90  00 

200,000  common  bricks,  at  $15  per  M.,  delivered 3,000  00 

7,000  Ibs.  wrought  iron  in  bolts,  braces,  etc.,  at  8  cts 560  00 

16,000  Ibs.  castings,  at  6  cts 960  00 

Labor  on  castings 700  00 

Iron  discharge  cars 125  00 

Freight  on  12,000  Ibs.  fire-bricks,  23,000  Ibs.  iron,  at  2  cts. .  700  00 

225  days'  mason  labor,  at  $6 .00 1,350  00 

180  days'  mason's  helper  labor,  at  $3 .00 540  00 

To  grading,  foundation,  sand,  centres,  and  scaffolding 600  00 

Superintendence  of  construction 400  00 


Total $9,025  00 

This  estimate  does  not  include  cost  of  conveyors  and  elevators,  and 
the  building  to  cover  the  furnace.  A  part  of  this  expense  belongs,  in 
reality,  to  the  pulverising  machinery.  These  items  will  vary  materi- 


96  BOASTING   FURNACES. 

ally  according  to  the  price  of  lumber.  From  $3,000  to  $4,000  is  a 
liberal  estimate  for  most  mining  districts. 

The  expense  of  roasting  in  the  Stetefeldt  Furnace  can  be  calculated 
as  follows : 

FUEL, — The  amount  of  fuel  required  in  24  h.  is  from  2  to  4  cords 
of  good  wood,  (or  its  equivalent  in  coal)  according  to  capacity  of 
furnace  and  character  of  ore.  Furnaces  of  20  to  25  tons  capacity, 
have  generally  consumed  from  2  J  to  2J  cords  of  nut  pine.  Only  well 
seasoned  wood  should  be  used  for  firing  the  furnace. 

LABOR. — Two  firemen  take  charge  of  the  furnace  in  24  h.,  in  12  h. 
shifts.  For  discharging  and  cooling  the  roasted  ore,  the  number  of 
men  required  is  in  proportion  to  the  amount  of  ore  roasted.  For 
instance,  a  furnace  of  25  tons  capacity  requires  four  men*  in  24  h,  to 
discharge  and  cool  the  pulp,  and  get  it  ready  for  amalgamation, 

SALT. — For  chloridizing  roasting  of  silver  ores,  the  Stetefeldt 
Furnace  requires  less  salt  than  any  other  furnace,  because  the  decom- 
position of  the  salt  is  very  perfect,  and  the  chlorine  and  chloridizing 
gases,  emanating  from  the  roasted  ore  at  the  bottom  of  the  shaft,  act 
upon  the  falling  ore,  which  floats  in  an  atmosphere  of  these  gases. 
Ores  which  are  free  from  lime  and  magnesia  can  be  chloridized  in  this 
furnace  with  a  minimum  of  salt.  The  results  of  a  run  of  nine 
months  of  the  Surprise  Valley  Mill,  Panamint,  OaL,  where  an  aver- 
age chlorination  of  92  to  93  per  cent,  was  obtained  in  roasting  silver 
ores,  of  $75  assay  value  per  ton,  with  only  2J  to  3  per  cent,  of  salt. 
In  fact,  a  still  lower  percentage  of  salt  would  have  been  sufficient, 
had  it  been  an  object  to  save  more.  In  case  the  ore  carries  lime  and 
magnesia,  or  a  larger  percentage  of  base  sulphurets,  it  is,  of  course, 
impossible,  even  in  the  Stetefeldt  Furnace,  to  obtain  good  chlorina- 
tions  with  such  a  slight  percentage  of  salt,  and  generally  from  5  to  8 
per  cent,  are  required.  The  salt  is  either  mixed  with  the  ore  on  the 
dry-kiln  and  both  crushed  together,  or  ore  and  salt  are  pulverized 


ROASTING  FURNACES.  97 

each  separately,  and  mixed  by  proper  machinery  before  entering  the 
Stetefeldt  feeder. 

The  cost  of  roasting  in  a  Stetefeldt  Furnace  of  25  tons  capacity, 
in  24  h.,  at  stated  prices  for  labor,  fuel,  and  salt,  such  as  are  generally 
paid  in  mining  districts  cf  Nevada,  are  calculated  as  follows : 

2  fireman,  at  $4.50. $  9  00 

4  pulp-coolers,  at  $4.00 16  00 

2f  corda  of  wood,  at  $8.00 22  00 

Wear  and  tear  of  screens,  etc 1  00 

Labor  and  fuel  for  25  tons $48  00 

Labor  and  fuel,  per  ton $1  92 

7  per  cent,  salt,  at  $40.00  per  ton 2  80 

Expense  of  chloridizing  roasting $4  72  per  ton. 

The  percentage  of  silver  chloridized  in  roasting  silver  ores  in  the 
Stetefeldt  Furnace  varies  according  to  the  character  of  ore,  and  the  care 
with  which  the  furnace  is  managed.  Results  as  high  as  97  per  cent, 
have  been  obtained,  while  the  average  chloridations  generally  range 
from  87  to  93  per  cent.  Ores  free  from  sulphur,  or  with  only  a  slight 
percentage  of  it,  should  be  mixed  with  one  or  two  per  cent,  of  pyrites 
of  iron,  otherwise  decomposition  of  the  salt  is  not  possible.  Oxidized 
ores,  however,  carrying  peroxides  of  manganese  and  iron,  which  give 
off  oxygen,  can  be  successfully  chloridized  by  themselves.  The  best 
results  are  obtained  by  mixing  sulphuret  ores  with  oxidized  ores, 
mainly  if  the  latter  contain  peroxide  of  manganese,  whereby  also  the 
capacity  of  the  furnace  is  much  increased. 

The  presence  of  copper  is  very  favorable  for  chlorination  of  the 
silver,  and  if  the  ore  is  of  such  a  character  that  it  bears  a  high  heat 
in  roasting  without  sintering,  the  chloride  of  copper,  formed  in  the 
upper  part  of  the  shaft,  can  be  almost  completely  decomposed,  and 
very  fine  bullion  produced  by  amalgamation.  As  an  example,  it  may 
be  stated  that  the  results  of  an  experiment  made  at  the  Surprise  Val- 
ley Mill,  Panamint,  Gal,  The  ore,  roasted  at  a  low  temperature,  gave 


98  BOASTINQ   FURNACES. 

bullion  only  300  to  400  fine  by  amalgamation,  the  base  metal  being 
copper.  By  roasting  the  same  ore  at  a  high  temperature,  the  bullion 
produced  was  almost  free  from  copper,  its  average  fineness  being  980 
during  a  run  of  nine  months. 

All  antimonial  ores  are  chloridized  with  great  facility,  and  with  a 
good  system  of  dust-chambers  the  loss  of  silver  by  roasting  is  hardly 
perceptible.  The  same  is  the  case  with  zincblende. 

A  Stetefeldt  furnace  was  put  in  operation  at  the  Ontario  Mill  in 
February,  1877.  The  Ontario  mine  carried  in  its  upper  levels  a  great 
amount  of  decomposed  ore,  which  yielded  a  very  high  percentage  of 
its  silver  by  raw  amalgamation.  In  the  lower  levels,  however,  the 
character  of  the  ore  changes  entirely,  and  the  ore  from  this  part  of  the 
mine  can  only  be  successfully  amalgamated  by  first  roasting  it  with 
salt.  After  reaching  the  500-foot  level  the  ore  became  very  much 
baser  than  that  met  with  before,  and  as  the  working  of  the  furnace 
gave  occasionally  much  inferior  results  than  those  formerly  obtained, 
an  analytical  investigation  was  ordered. 

Specimens  from  the  500-foot  level  of  the  Ontario  plainly  show  the 
following  minerals,  viz :  zincblende,  galena,  fahlore,  pyrites  of  iron. 
Of  the  zincblende  there  are  two  varieties,  the  one  of  a  light  yellow 
color,  the  other  black.  In  the  latter  a  larger  percentage  of  fahlore  is 
found  between  the  cleavages  than  in  the  former.  Also,  the  galena  is 
often  intimately  mixed  with  fahlore.  The  fahlore  itself  occurs,  be- 
sides, in  compact  masses. 

A  chemical  analysis  of  an  average  sample  of  crushed  ore,  made  by 
Stetefeldt,  gave  the  following  results : 

One  hundred  parts  of  the  ore  contain : 
9.60  zinc, 
6.07  lead, 
2.77  iron, 
1.41  copper, 


BOASTING  ffUKNACEfl.  99 

0.45  manganese, 
0.60  silver, 
7.68  sulphur, 
1.20  antimony, 
0.20  arsenic, 
55.21  silica, 
13.14  alumina, 

1.00  potassa  and  soda  from  decomposed  feldspathic  matter. 
Traces  of  bismuth,  cadmium,  lime,  magnesia. 
Assay  value,  $229.34  silver  per  ton. 

The  silver-bearing  minerals,  after  eliminating  the  fahlore  from  the 
zincblende  and  galena,  were  found  to  be  composed  as  follows  : 
Yellow  zincblende  contains: — 1.60  per  cent,  iron, 

0.40       "        copper, 
Trace  cadmium. 
Assay  value,  $92.50  silver  per  ton. 

Black  zincblende  contains: 2.40  per  cent,  iron, 

0.84       «        copper, 

Trace  cadmium. 
Assay  ralue,  $92.50  per  ton. 

Galena  contains 1.80  per  cent,  iron,  . 

0.66      "        copper, 
Trace  bismuth. 
Assay  value,  $175.00  silver  per  ton. 

The  fahlore  contains 24.16  per  cent,  copper, 

11.77       "        silver, 
6.70       "        zinc, 
4.20      «        lead, 
27.00      "        antimony, 
4.40       "        arsenic, 
22.20      "        sulphur. 
Assay  value,  $4,440.00  silver  per  ton. 


100  BOASTING  FURNACES. 

The  fahlore  is  the  main  source  of  the  silver  in  the  Ontario  ore.  It 
is,  of  course,  more  or  less  intimately  mixed  with  the  galena  and  zinc- 
blende,  but  in  crushing  the  ore  through  a  !N"o.  60  screen,  the  fahlore 
becomes  sufficiently  free  to  allow  its  roasting  independently  of  the 
minerals  with  which  it  is  associated.  Ore  of  exactly  the  same  char- 
acter as  analyzed  was  roasted  with  about  13  per  cent,  of  pure  salt  in 
the  Stetefeldt  furnace,  and  samples  taken  from  the  shaft  and  flue. 
These  were  subjected  to  a  chemical  analysis.  The  object  of  this  in- 
vestigation was  to  ascertain  if  there  is  a  sufficient  quantity  of  salt 
falling  down  the  shaft,  as  the  chlorinations  there  have  been  recently 
often  very  low ;  then  whether  undecomposed  salt  is  left  in  the  roasted 
ore,  and  how  much,  also  which  of  the  base  metals  have  been  changed 
to  chlorides  and  which  to  sulphates ;  finally,  how  much  sulphur,  not 
changed  to  sulphuric  acid,  has  been  left  in  the  roasted  ore  in  combi- 
nation with  metals  as  sulphurets. 

The  solution  of  these  questions  must  give  an  exact  knowledge  of 
the  working  of  the  furnace. 

The  sample  of  roasted  ore  from  the  shaft  waa  found  to  contain  in 
100  parts: 

0.25  chloride  of  copper, 

1.51  chloride  of  aluminium, 

1.38  chloride  of  zinc, 

3.68  chloride  of  sodium, 

Traces  of  chlorides  of  other  metals, 

3.26  sulphate  of  lead, 

0.56  sulphate  of  alumina, 

4.62  sulphate  of  soda, 

Traces  of  sulphates  of  other  metals, 

Rest,  metallic  oxides  and  gangue. 

Of  the  silver  contained  in  the  ore,  81.32  per  cent,  were  chloridized. 
Sulphur  in  undecomposed  sulphurets,  0.18  per  cent, 


ROASTING   FURNACES.  101 

The  sample  of  roasted  ore  from  the  flue  was  found  to  contain  in 
one  hundred  parts : 

1.07  chloride  of  aluminium, 

3.08  chloride  of  sodium, 

Traces  of  chlorides  of  other  metals, 
0.74  sulphate  of  copper, 
2.88  sulphate  of  alumina, 
1.48  sulphate  of  zinc, 
5.18  sulphate  of  lead, 
10.01  sulphate  of  soda, 
Traces  of  sulphates  of  other  metals, 
Rest,  metallic  oxides  and  gangue. 

Of  the  silver  contained  in  the  ore  82.24  per  cent,  were  chloridized. 
Sulphur  in  undecomposed  sulphurets,  0.064  per  cent. 

These  results  are  very  interesting,  and  prove: — 

1  «.• 

1.  That  in  the  shaft,  as  well  as  in  the  flue,  over  three  per  cent,  of 

undecomposed  salt  are  present,  and  that,  consequently,  an  increase  of 
salt  in  roasting  would  be  useless. 

2.  That  there  remains  more  undecomposed  salt  in  the  shaft  than 
in  the  flue,  and  that  the  low  results  in  the  chlorination  of  the  silver 
frequently  obtained  of  late  in  the  shaft  cannot  be  accounted  for  by  an 
insufficient  quantity  of  salt  dropping  down  the  shaft.     Consequently, 
it  would  be  of  no  benefit  to  crush  the  salt  by  itself  through  a  coarser 
screen. 

3.  That  the  volatilization  of  salt  is  by  no  means  as  considerable  as 
supposed. 

4.  A  peculiar  relation  exists  between  the  shaft  and  flue  in  regard  to 
the  salts  of  copper,  zinc,  and  aluminium.     Copper  and  zinc  are  present 
in  the  shaft  only  as  chlorides,  in   the   flue  only  as   sulphates.     The 
aluminium  exists  in  the  shaft  as  well  as  in  the  flue  in  both  combina- 


102  BOASTING   FURNACES. 

tions,  but  in  the  shaft  the  chloride  is  predominant,  and  in  the  flue  the 
sulphate.  It  is  evident  that  in  the  shaft  the  metallic  sulphates  are 
more  completely  decomposed,  as  the  ore,  falling  against  the  rising 
gases,  is  longer  suspended  in  the  heat,  and  passes  finally  through  the 
hottest  zone  of  the  furnace.  In  the  flue  the  ore  particles  move  with 
the  gases,  are  only  for  a  moment  exposed  to  the  flame  of  the  auxiliary 
fireplace,  and,  being  instantaneously  transferred  to  a  lower  tempera- 
ture, the  sulphates,  once  formed,  have  no  occasion  to  lose  their  acid, 
except  by  contact  in  the  charge  with  salt,  which  is,  necessarily,  a  slow 
and  imperfect  mode  of  decomposition.  The  presence  of  these  sul- 
phates has,  however,  a  beneficial  effect  upon  a  further  chlorination  of 
the  silver  in  the  ore  at  the  bottom  of  the  flue. 

As  to  the  lead,  we  find  it  only  as  a  sulphate  in  the  shaft  as  well  as 
in  the  flue,  but  the  preponderance  of  the  formation  of  sulphates  in  the 
flue  becomes  again  evident.  As  the  sulphate  of  lead  does  not  lose  its 
acid  at  a  high  temperature,  it  follows  that  its  decomposition  in  the 
shaft  cannot  be  accomplished. 

5.  The  complete  oxidation  of  the  sulphur  is  very,  surprising,  and 
shows  how  thoroughly  the  furnace  does  its  work  even  with  such  base 
ores. 

6.  Chlorinations  of  eighty-one  and  eight-two  per  cent,  of  the  silver 
are  very  high  for  Ontario's  ore,   and   with   such  chlorinations   from 
ninety  to  ninety-four  per  cent,  of  the  silver  ore  is  extracted  by  amal- 
gamation, without  any  difficulty. 

The  results  of  this  investigation  have  clearly  demonstrated  that  ores 
like  those  from  the  500-foot  level  of  the  Ontario  mine,  can  be  worked 
with  perfect  success  by  the  Stetefeldt  furnace,  if  proper  conditions  are 
maintained.  Inferior  results  were  only  caused  by  a  deficiency  in  the 
draught  of  the  furnace,  or  in  overfeeding  it  when  the  draught  was  not 
in  a  normal  condition.  •  These  defects  can  be  easily  corrected. 


III.    EXTRACTION  OF  SILVER 

BY  WAY  OF  LIXIVIATION. 


To  lixiviate,  or  to  leach  a  soluble  metal  out  of  ore — that  is,  to  filter 
a  liquid  through  the  ore,  so  that  it  dissolves  and  carries  out  the  metal 
in  a  clear  solution — is  the  process  called  the  lixiviation,  or  leaching 
process.  If  there  is,  for  instance,  sulphate  of  copper,  and  sulphate  or 
chloride  of  iron  in  the  ore,  the  water,  passing  through  it,  dissolves 
the  soluble  copper  and  iron  salts,  and  takes  it  out  of  the  ore;  but  it 
does  not  dissolve  the  chloride  of  silver;  therefore  this  remains  with  the 
ore  in  the  leaching  tub.  In  order  to  leach  now,  also,  the  silver, 
another  liquid  must  be  employed,  which  has  the  property  of  dissolv- 
ing the  chloride  of  silver.  Chloride  of  silver  is  the  only  silver  com- 
bination that  can  be  successfully  lixiviated.  It  is  therefore  a  primary 
condition  that  the  ore  should  be  subjected  to  a  chloridizing  roasting, 
before  the  leaching  process  can  be  made  use  of.  No  silver  ore,  except 
natural  chlorides  or  bromides,  is  suitable  for  lixiviation  in  a  raw  state. 

There  are  different  solutions  which  dissolve  the  chloride  of  silver. 
A  hot  concentrated  solution  of  common  salt,  a  solution  of  hyposul- 
phite of  soda  or  of  lime,  ammonia,  etc.;  all  dissolve  the  chloride, 
and  filter  with  it  as  a  clear  solution,  out  of  which  it  must  be  precipi- 
tated. There  are  again  different  ingredients  by  which  this  can  be 
effected.  The  silver  is  precipitated  by  polysulphide  of  sodium,  or  of 
calcium,  also  by  sulphureted  hydrogen,  which  is  cheaply  produced  by 
melting  in  a  porcelain  or  glass  vessel  parafin,  with  flower  of  sulphur. 
In  all  these  cases  the  silver  is  precipitated  as  a  sulphide,  or  in 
metallic  condition  by  metallic  copper, 


104  LIXIVIATION. 

The  extraction  of  the  chloride  of  silver  by  alkaline  hyposulphites 
was  proposed  by  Percy.  Patera  was  the  first  who  made  use  of  the 
hyposulphite  of  soda  for  extraction  of  silver  in  a  practical  way;  his 
success,  however,  depends  principally  on  his  modified  and  complicated 
roasting.  By  lixiviation  the  silver  is  extracted  in  the  Patera,  Kiss, 
Roszner-Patera,  Ziervogel  and  Augustin  processes. 

The  extraction  of  silver  by  the  solving  process  is  simple.  The  ore 
is  first  roasted  with  salt  in  the  usual  way,  whereby  the  formation  of 
base  metal  chlorides  cannot  be  avoided  entirely.  After  roasting,  the 
ore  is  first  subjected  to  leaching  with  water,  in  order  to  extract  the 
base  metal  chlorides,  and  then  with  hyposulphite  of  lime,  to  extract 
the  silver. 

The  belief  that  by  the  leaching  process  a  purer  bullion  can  be  ob- 
tained than  by  amalgamation  is  erroneous.  If  there  is  a  great  deal 
of  lead  in  the  ore,  which  in  roasting  changes  into  sulphate  of  lead, 
this  will  not  be  amalgamated  in  pans,  while  all  of  it,  may  it  be  as  a 
chloride  or  as  a  sulphate,  will  be  dissolved  by  the  hyposulphite  and 
carried  out  with  the  silver. 

The  only  way  under  such  circumstances  to  get  a  fine  bullion  by  the 
leaching  process,  is  explained  further  on  below. 

The  Extraction  of  Silver. 

After  a  chloridizing  roasting  the  ore  should  be  examined  to  ascer- 
tain the  amount  of  chloride  of  silver^contained  in  it  (page  32.)  In 
case  the  extraction  should  not  be  satisfactory,  it  is  then  easier  to  find 
what  the  cause  is. 

The  leaching  vat  is  best  made  round,  as  it  is  much  easier  to  make 
the  staves  fitting,  water  tight,  than  large  square  boxes.  Large  vats 
can  be  made  also  of  masonry,  lined  with  asphaltum.  The  bottom  in 
this  case  must  be  made  very  carefully.  In  order  to  permit  the  filtra- 
tion, there  must  be  a  false  bottom  in  each  vat  or  box.  This  false 


LIXIVIATION. 


105 


bottom  is  prepared  in  different  ways.  On  the  bottom  are  laid 
pebble  stones  of  the  size  of  a  hen's  egg,  then  on  this  a  layer  of 
smaller  ones,  and  so  on  till  all  is  covered  with  sand  free  of  inud ;  this 
false  bottom  is  4  or  5  inches  deep.  The  ore  to  be  leached  is  thrown 
direct  on  this  filter.  A  better  way  is  to  put  wooden  staves  of  about 
one  inch  in  thickness,  as  represented  in  figure  14. 
.  Fig.  u.  • 

The  first  staves,  a,  on  the 
bottom,  are  12  inches  apart, 
and  on  the  front  side  where 
the  outlet,  x,  is,  the  staves 
should  not  touch  the  side,  so 
that  the  solution  can  flow  to 
the  outlet,  as  shown  by  the 
arrows.  The  second  row  of 
staves  is  layed  across  the  first 
ones,  one  inch  apart.  The 
whole  is  then  covered  with  a 
strong,  but  not  too  tight,  cot- 
ton cloth,  d,  or  some  other 
stuff  like  gunny  sack,  cocoa 
matting,  etc.  It  happens, 
that  when  water  is  admitted, 
this  breaks  through  in  one 
or  the  other  place,  and  carries 
out  ore.  This  can  be  effect- 
ually prevented  by  fixing 
above  the  staves,  on  all  four 
sides,  a  slat  of  four  inches 
wide,  c,  cut  as  represented  in 
the  drawing.  The  ends  of 
the  wet  cloth  are  pressed  into 


106  LlXlVIATION. 

the  space  formed  by  the  beveled  cut.  The  size  of  leaching  vats  is 
generally  from  8  to  10  feet  in  diameter;  or,  if  rectangular  tanks  are 
preferred,  they  can  be  made  as  large  as  12  by  15,  or  12  by  12  feet. 
The  last  sized  tank  would  then  contain  144  square  feet,  and  if  the  ore 
is  charged  one  foot  deep,  there  are  so  many  cubic  feet.  The  roasted 
ore  generally  weighs  from  65  to  75  Ibs.  per  cubic  foot,  and,  calculating 
the  average  with  70  Ibs.,  it  would  require  about  ten  tons  of  roasted 
ore  to  charge  a  like  tank ;  but  some  ore  allows  nitration  when  two  feet 
deep,  and  in  this  case  the  charge  would  be  doubled.  From  bricks, 
lined  with  asphaltum,  such  tanks  can  be  made  large  enough  for  ten  to 
twenty  tons  of  ore  at  a  charge.  The  hight  of  the  sides  must  corres- 
pond with  the  nature  of  the  ore.  Some  of  it  niters  freely  if  25  inches 
deep,  but  other  kinds  of  ore  hardly  allow  filtering  if  it  is  charged  only 
12  inches  deep.  There  should  always  be  at  least  12  inches  space  above 
the  charge  to  receive  the  solution.  In  the  outlet,  d,  an  india  rubber 
hose  of  two  or  three  feet  in  length  is  tightly  inserted,  and  the  end  of 
it  hung  up,  as  shown  in  the  sketch. 

Generally,  the  vat  is  charged  with  the  roasted  ore  without  sifting, 
and  the  surface  spread  evenly,  leaving  about  six  inches  -space  from  the 
top.  The  water  is  now  introduced,  either  hot  or  cold,  and  if  the 
filtering  goes  very  slow,  hot  water  will  help  a  great  deal.  It  sinks 
slowly  towards  the  bottom,  and  the  air  escapes  through  the  hose. 
After  the  ore  is  covered  several  inches  with  water,  the  hose  is  discon- 
nected and  placed  in  the  trough  which  conveys  the  lixivium  to  the 
precipitating  tubs. 

In  the  beginning,  the  leaching  water,  as  it  escapes  through  the 
hose,  is  highly  charged  with  base  metal  salts,  and  shows  a  green  color 
if  there  is  much  copper  in  the  ore.  The  water  is  kept  running 
continuously,  and  the  influx  and  the  efflux  are  equalized.  After  one 
or  two  hours  a  glass  full  of  the  liquid,  at  the  hose,  is  taken,  and  a 
few  drops  of  sulphide  of  calcium  (or  of  sodium)  added.  If  a  precipi- 


LIXIVIATION.  107 

tate  falls,  of  a  dark  or  light  color,  the  leaching  must  continue;  but  it 
is  not  necessary  to  continue  until  no  precipitate  at  all  is  perceived,  as 
it  requires  some  time — perhaps  an  hour,  before  all  the  water  runs  out. 
The  water  which  comes  out  last  must  be  free  from  salts.  This  first 
leaching  takes  from  two  to  four  hours,  sometimes  longer. 

As  soon  as  the  ore  is  freed  from  the  base  chlorides  soluble  in  water, 
a  solution  of  hyposulphite  of  lime  or  in  the  commencement,  a  solu- 
tion of  hyposulphite  of  soda  is  led  in  from  a  tub  or  tank,  on  the  ore, 
in  order  to  dissolve  the  chloride  of  silver.  This  leaching  is  conducted 
like  the  former.  It  depends  on  the  amount  of  silver  how  long  this 
work  continues — from  eight  to  twenty  hours.  The  clear  cold  solution, 
containing  the  chloride  of  silver  in  the  form  of  a  double  salt,  has  a 
very  sweet  taste,  and  is  conveyed  through  a  trough  or  india  rubber 
hose  into  a  precipitating  tub.  Very  rich  ore,  containing  from  1 2  to 
15  per  cent,  of  silver,  would  require  forty-eight  hours  leaching,  and 
even  then  it  would  be  necessary  to  subject  the  ore  to  a  second  leach- 
ing with  the  hyposulphite,  with  an  intermediate  roasting  with  green 
vitriol  and  salt;  for,  with  the  best  work,  if  95  per  cent,  are  extracted, 
the  tailings  would  still  appear  sufficiently  rich  for  this,  containing 
about  200  ounces  of  silver  per  ton.  Ores  containing  $350  per  ton 
are  often  leached  out  perfectly  in  twelve  hours.  The  end  of  the 
lixiviation  is  ascertained  in  the  same  way  as  in  leaching  with  water, 
using  the  sulphide  of  calcium.  If  no  precipitate  is  obtained  the 
extraction  is  finished. 

The  tailings,  as  they  are  now  in  the  vat,  contain  a  great  deal  of 
hyposulphite  solution,  which  should  not  be  lost.  Water  is  therefore 
turned  again  on  the  tailings  to  displace  the  solution,  which  is  conveyed 
into  one  of  the  precipitating  tubs,  as  long  as  there  is  a  distinct  taste 
in  it,  and  in  the  same  way,  when  leached  with  water  before  the  silver 
was  extracted;  the  water  must  be  displaced  by  the  hyposulphite  before 
the  solution  is  turned  into  the  precipitating  tubs.  This  part,  however, 


108  LIXIVIATION. 

must  be  carefully  attended  to,  not  -to  miss  the  time  when  the  silver 
begins  to  come  out;  a  sweetish  taste  indicates  the  time,  or  a  slight 
precipitation  with  hyposulphide  of  calcium. 

The  color  of  the  precipitate  is  a  black-brown.  The  presence  of  base 
metals  changes  the  color  somewhat.  Iron  makes  it  black ;  copper,  red 
brown ;  lead  and  antimony,  light  red-brown,  etc.  The  silver  is  first 
dissolved,  especially  if  a  diluted  solution  of  hyposulphite  of  lime  is 
used,  and  for  this  reason  the  first  precipitate  is  the  richest  in  silver. 
Ore  containing  a  great  deal  of  lead — especially  if  the  roasting  was  so 
conducted  that  a  large  part  of  it  remained  as  sulphate  of  lead,  which 
is  not  soluble  in  the  leaching  water — will  give  in  the  beginning  of  the 
leaching  with  the  solvent  a  precipitate  of  silver  with  some  lead ;  after- 
wards, however,  the  silver  diminishes,  so  that  the  precipitate  of  lead 
finally  appears  free  of  silver.  Besides  the  sulphate  of  lead,  sub-chlo- 
rides and  oxy-chlorides  are  formed  during  the  roasting,  which  are  not 
soluble  in  water,  but  are  dissolved  by  the  hyposulphite  of  lime ;  for 
this  reason  always  some  base  metals  will  be  found  in  the  precipitate. 

In  case  rebellious  ores  are  treated,  and  hot  water  is  used  for  the 
extraction  of  base  chlorides,  a  better  silver  is  obtained  if  the  ore  is 
cooled  down  by  cold  water  before  the  cold  and  diluted  solvent  is 
applied.  Purer  ores  may  be  treated  with  a  warm  solution  of  the 
solvent. 

When  examining  the  tailings  as  to  the  amount  of  silver  left  there- 
in, as  compared  with  the  original  ore  after  roasting,  it  must 
be  remembered  that,  after  leaching  out  a  quantity  of  metals 
by  water  and  the  solvent,  the  ore  lost  a  considerable  part  of  its  origi- 
nal weight,  and  that,  consequently,  one-half  ounce  of  such  tailings 
taken  into  assay  will  always  give  a  larger  silver  button  than  there 
ought  to  be.  A  true  assay  of  leached  tailings  is  made  if  half  an  ounce 
of  the  same  ore  is  leached  on  a  filter  with  hot  water  and  hyposulphite 
of  lime,  in  the  same  way  as  the  ore  on  a  large  scale,  washed  with 
water,  dried  and  weighed.  The  weight  found  after  leaching  must  be 
taken  for  half  an  ounce  in  assaying  the  tailings. 


LIXIVIATION.  109 

The  residue,  or  tailings  in  the  leaching  box,  must  be  removed  now 
as  valueless.  The  sides  of  the  leaching  boxes  are  from  eighteen  to 
twenty-four  inches  above  the  bottom,  and'  being  from  six  to  eight 
feet  square  in  the  clear,  the  removing  of  the  tailings  by  means  of 
shovels  is  easily  effected.  The  men  must  be  careful  not  to  dig  too 
deep,  otherwise  the  filter  will  be  injured.  It  is  quite  proper  to  fix 
wooden  staves,  as  long  as  the  box  requires,  on  top  of  the  filter.  These 
staves  are  one  inch  wide  and  one-half  of  an  inch  thick,  and  are  placed 
from  four  to  five  inches  apart,  so  as  to  protect  the  canvas  or  filter 
against  the  shovel. 

If  the  ore  is  very  base,  the  greatest  part  of  the  silver  can  be 
obtained  over  800  fine,  if  the  solution  is  not  strong  (J°  Bau.).  When 
the  leaching  commences,  the  solution  appears  sweetest  from  dissolved 
silver.  After  a  while,  when  the  sweet  taste  disappears,  the  leaching 
may  be  continued  for  one-quarter  or  half  an  hour,  and  then  the  follow- 
ing solution  conveyed  into  another  precipitating  tub.  This  last 
precipitation  will  give  a  very  low  bullion,  which  should  be  either 
cupelled  with  lead,  or  if  too  poor,  dried  and  roasted  again  with  ore. 
The  time  when  the  lixiviation  should  cease,  is  easily  found  out.  With 
the  watch  in  hand,  the  solution  from  the  leaching  vat  is  led  in  a 
bucket,  or  better  into  a  pitcher,  for  exactly  one  minute.  The  silver 
and  other  metals  are  precipitated  by  sulphide  of  calcium,  collected 
carefully  in  a  filter,  washed  with  water,  and  then,  after  drying  thor- 
oughly, melted  with  litharge  in  a  crucible  and  cupelled.  Sixty  times 
the  weight  of  the  resulting  button  gives  the  amount  of  silver  per 
hour  that  is  leached  out.  A  calculation  for  the  day  shows  whether  it 
covers  the  expenses  of  labor. 

There  is  another  important  fact  connected  with  the  lixiviation. 
The  different  chlorides,  being  removed  in  the  first  leaching  with  water, 
are  principally  those  of  copper,  iron,  lead,  antimony  and  zinc,  besides 
some  undecomposed  salt.  The  first  quantity  of  water  introduced 


110  LIXIVIATION. 

into  the  leaching  box  is,  of  course,  most  saturated  with  the  named 
salts,  and  they  have  the  property  of  dissolving,  also,  some  chloride  of 
silver.  The  dissolved  silver  precipitates  again  as  soon  as  it  becomes 
diluted  with  more  water.  There  is,  therefore,  no  difficulty  in  regain- 
ing the  silver  which  is  thus  leached  out.  The  amount  of  silver  carried 
out  by  the  leaching  water  varies  from  0.5  to  three  per  cent.  Not 
only  the  chloride  of  silver,  but  also  those  of  lead  and  antimony,  are 
precipitated  by  dilution  with  water.  There  are  two  ways  of  regaining 
this  silver. 

Mr.  0.  Hofmann  adopted  an  ingenious  plan  for  this  purpose,  by 
conveying  the  hot  water,  under  a  slight  pressure  from  below,  under 
the  false  bottom,  as  described  in  (page  124.) 

The  other  plan  is  the  precipitation  of  the  silver,  together  with  the 
chlorides  of  lead  and  antimony,  outside  of  the  leaching  box.  This 
mode  is  preferable  to  the  former  when  a  great  deal  of  lead  and 
antimony  is  in  the  ore;  for  if  precipitated  in  the  box,  all  of  it  will 
be  dissolved  by  the  hyposulphite  of  lime,  and  then  precipitated  as 
sulphides  with  the  silver,  making  this  impure  and  consuming  much  of 
the  precipitating  agent.  As  soon  as  the  dissolved  chlorides  flow  into 
the  trough,  below  the  leaching  vat,  into  which  several  leaching  boxes 
discharge  their  fluids  in  different  degrees  of  dilution,  the  gradual 
precipitation  commences.  An  additional  small  stream  of  clear  water 
will  hasten  the  precipitation,  which  is  white  and  adheres  to  the  trough 
through  the  whole  length  of  it.  These  chlorides  are  the  richest,  and 
contained,  at  Flint,  Idaho,  9  per  cent,  of  silver ;  the  balance  was  prin- 
cipally lead  and  antimony.  The  precipitate  deposits  on  all  bodies 
offering  a  surface.  For  this  purpose  a  box  must  be  constructed,  the 
sides  of  which  are  six  inches  high.  There  are  partitions  four  inches  high, 
leaving  a  space  of  six  inches  between  them,  so  that  the  water  must 
flow  from  the  first  partition,  which  on  one  side  does  not  reach  the  six- 
inch  side  of  the  box,  into  the  second,  and  from  this  on  the  other  side 


Ill 

into  the  third  partition,  and  so  on  in  a  zigzag  way.  The  space  between 
the  partitions  is  filled  with  shavings,  offering  a  large  amount  of  sur- 
face for  the  chlorides  to  deposit  thereon. 

The  water  leaving,  the  box  contains,  principally,  copper  and  iron  in 
solution. 

The  white  precipitate,  when  accumulated,  is  taken  out,  placed  in 
filtering  bags,  with  or  without  the  shavings,  and  washed  with  clear 
cold  water,  in  order  to  get  rid  of  the  copper  solution.  The  silver  can 
be  extracted  in  two  ways :  The  simplest  mode  is  the  application  of 
hyposulphite  of  lime.  The  sediment  is  taken  out  from  the  filtering 
bags  and  charged,  while  wet,  into  a  filtering  box  of  a  proper  size.  The 
hyposulphite  of  lime,  in  a  cold  condition,  is  poured  over  it  and  man- 
aged like  the  ore.  The  silver-holding  fluid  may  be  conveyed  into  the 
precipitating  tub  and  treated  with  the  solution  from  the  ore.  The 
liquid  from  the  filtering  box  is  examined  from  time  to  time  with  sul- 
phide of  calcium.  In  the  beginning  the  precipitate  appears  dark, 
being  mostly  silver;  but  when  it  is  perceived  that  the  precipitate 
assumes  a  light  yellow  color,  too  much  of  lead,  zinc  and  antimony 
is  being  carried  out,  and  the  leaching  must  be  stopped.  The  residue 
in  the  filter  box  contains  still  some  silver. 

The  other  mode  of  extraction  is  more  perfect,  but  also  more  expen- 
sive and  more  troublesome.  After  the  copper  has  been  washed  off, 
the  contents  of  the  bags  are  taken  out  and  dried.  It  is  then  intro- 
duced into  large  crucibles  and  smelted  with  an  addition  of  soda-ash. 
The  reduced  metal,  if  some  lead  occurs  in  the  ore,  must  be  separated 
by  means  of  cupellation,  resulting  in  clean  silver  and  litharge. 

The  chloride  of  copper,  after  the  silver  with  the  lead  deposited  in  the 
trough  and  box,  is  led  into  a  reservoir  in  which  old  iron  is  suspended. 
The  copper  precipitates  in  a  metallic  state  on  the  iron,  and  about 
eighty-eight  parts  of  iron  go  into  the  solution  in  place  of  one  hundred 
parts  of  copper ;  consequently,  as  each  one  hundred  pounds  of  pure 


112  LIXIVIATION. 

copper  require  eighty-eight  pounds  of  iron,  the  calculation  as  to  the 
necessary  amount  of  iron  could  be  made  easily  if  it  were  not  for  some 
other  chlorides  which  may  still  be  in  solution,  and  which  also  require 
iron  for  precipitation.  Wrought-iron  is  preferable  to  cast-iron,  and 
gray  cast-iron  is  better  than  white ;  but  all  these  sorts  precipitate  the 
copper,  and  it  depends  to  a  great  extent  on  the  price  as  to  what  kind 
of  iron  is  chosen. 

The  most  effective  precipitant  is  the  iron-sponge  or  finely  divided 
iron,  obtained  by  heating  pulverized  iron  ore  or  roasted  iron  pyrites 
with  charcoal  powder  in  a  proper  reverberatory  furnace,  or  in  iron 
pipes  or  cylinders  without  admitting  air.  By  these  means  the  iron 
oxide  is  reduced  to  metallic  iron,  which  precipitates  the  copper  in  a 
few  minutes.  Using  old  iron,  the  precipitation  will  be  effected  much 
more  quickly  than  in  tanks,  if  the  copper-holding  solution  is  conveyed 
into  a  revolving  barrel  containing  iron,  and  heated  up  by  steam,  or 
perhaps  still  better,  into  a  box  in  which  a  perforated  barrel  revolves, 
containing  old  iron.  The  precipitated  copper  would  constantly  fall 
through  the  holes  of  the  barrel  into  the  box,  being  kept  thas  separately 
from  the  iron. 

To  find  out  whether  there  is  yet  copper  in  the  liquid,  the  best  test 
is  to  take  some  drops  on  a  piece  of  platinum,  and  to  place  a  small, 
clean  piece  of  zinc  on  it.  The  copper  immediately  appears  of  a  bright 
red  color.  But  for  practical  use,  a  clean  piece  of  iron  dipped  into  the 
liquid  will  also  show  a  red  coating,  if  there  is  enough  copper  in  it  to 
make  it  remunerative  to  continue  the  precipitation.  The  water  con- 
tains now  principally  chloride  of  iron,  and  is  discharged.  If  by  some 
cheap  means  the  water  could  be  evaporated,  the  remaining  chloride  of 
iron  could  be  used  in  roasting  ores  without  salt. 

Where  old  iron  commands  a  high  price,  the  copper  can  be  precipi- 
tated with  a  brine  of  ashes  or  of  lime,  as  carbonate  of  copper;  but  in 
this  case  the  iron  also  falls  with  the  copper.  The  brine  for  this  reason 


LIXIVIATIOX.  113 

cannot  be  advantageously  adopted  where  a  great  deal  of  iron  is  in  the 
ore,  or  the  roasting  must  be  directed  so  as  to  decompose  the  chloride 
of  iron. 

If  the  solution  of  hyposulphite  of  lime  is  not  managed  properly, 
for  instance,  by  careless  precipitation  of  the  silver^  leaving  too  much 
metal  not  precipitated  in  it,  or  by  introducing  the  solution  on  the  ore, 
without  leaching  it  first  with  water,  augmenting  by  these  means  salt 
and  sulphate  of  soda  in  the  solution  of  hyposulphite  of  lime,  this  solu- 
tion refuses  to  dissolve  the  silver  as  well  as  it  did  in  the  beginning, 
and  requires  then  to  be  renewed,  which  is  expensive  and  troublesome. 
To  revive  a  solution  of  this  description,  it  is  sufficient,  when  the  vat 
is  charged,  to  spread  wood  ashes  over  the  entire  surface  of  the  ore, 
two  or  three  inches  deep,  and  then  to  admit  the  solution.  It  will 
now  again  act  like  a  new  solution. 

Precipitation  of  the  Silver. 

The  brine  containing  the  silver,  obtained  from  the  leaching  tanks, 
flows  through  the  trough  or  rubber  hose  into  the  precipitating  vats. 
These  are  from  five  to  six  f ee.t  high  and  four  feet  in  diameter.  Figure 
15  represents  a  precipitating  vat.  The  bottom,  a,  is  laid  out  with 
Roman  cement,  and  the  sides  inclining  towards  the  large  brass  cock, 
through  which  the  precipitated  silver  is  discharged  directly  into  the 
filter  below,  the  filter  being  two  feet  wide  by  three  or  four  feet  long, 
and  from  ejght  to  ten  inches  deep.  The  solution  that  runs  through 
the  filter  falls  into  the  trough,  b. 

According  to  the  capacity  of  the  works,  there  must  be  from  four  to 
eight  of  these  vats.  When  the  solution,  coming  from  the  leaching 
vats,  rises  to  within  eighteen  or  twenty  inches  from  the  rim  of  the 
vat,  the  flow  is  turned  into  the  next  vat,  and  the  precipitation  of  the 
first  one  commences  immediately.  For  this  purpose  a  bucketfull  of 
the  sulphide  of  calcium  is  poured  into  the  vat,  and  the  silver  precip- 
itated. The  solution  is  then  stirred  vigorously. 


114 


LIXIVIATION. 


Fig.  15. 


Treating  always  the  same  kind  of  ore,  the  required  quantity  of  the 
precipitating  agent  is  soon  learned.  The  black  precipitate  sinks  to 
the  bottom,  and  the  workman  now  dips  a  little  of  the  dear  liquid  out 
in  a  glass  tube  or  tumbler,  and  adds  a  few  drops  of  the  sulphide  of 
lime.  If  a  dark  precipitate  or  a  dark  color  is  produced,  it  shows  that 
there  is  still  silver  in  the  liquid,  and  more  of  the  agent  must  be  added; 
but  if,  on  the  contrary,  no  precipitate  is  observed,  there  is  either 
enough  or  too  much  of  the  sulphide.  To  prove  this,  some  of  the  silver- 
holding  liquid  is  added  to  a  test,  taken  from  the  other  tank  under  treat- 
ment. If  in  this  case  a  precipitate  is  formed,  silver-holding  liquid  must 
be  carefully  added  to  the  liquid  in  the  tank  until  no  reaction  is  pro- 


LIXIVIATION.  115 

duced.  This  work,  delicate  as  it  seems,  is  easily  learned  by  the  workmen. 
If  a  little  silver  should  be  left  in  the  liquid,  it  is  not  injurious,  neither 
is  the  silver  to  be  considered  as  lost,  because  the  same  liquid  is  used 
over  again ;  but  a  small  excess  of  the  sulphide  of  calcium  would  cause 
a  loss  in  silver,  as  it  precipitates  sulphide  of  silver  in  the  leaching 
tank  in  the  mass  of  ore,  which  is  not  dissolved  again.  The  precipita- 
tion is  performed  in  a  short  time,  requiring  about  fifteen  minutes  for 
each  tank.  The  stirring  must  be  executed  with  vigor.  Wooden 
grates  fixed  to  a  vertical  stem  will  answer  the  purpose.  Some  use 
perforated  wooden  disks  on  the  stem.  After  the  last  portion  of  the 
precipitant  had  been  added  and  well  stirred,  the  precipitated  silver 
will  soon  settle;  but  it  is  well  to  allow  one  hour's  time  at  least.  The 
liquid  must  be  clear.  All  the  sulphide  of  calcium  that  was  used,  by 
giving  up  a  part  of  its  sulphur  to  the  silver,  is  reduced  to  hyposul- 
phite of  lime,  and  in  this  way  the  amount  of  hyposulphite  is  constantly 
increasing;  but  on  the  other  hand  there  is  also  some  waste  occurring 
at  the  end  of  the  leaching.  If  the  sulphide  of  calcium  is  made  too 
weak,  the  hyposulphite  of  lime  becomes  diluted  by  degrees,  and  the 
lixiviation  would  be  imperfect. 

The  clear  solution  above  the  settled  precipitate  must  be  drawn  off, 
either  with  a  rubber  syphon,  to  one  end  of  which  two  short  pieces  of 
wood  are  fastened,  as  shown  in  figure  15,  c,  so  as  to  prevent  the  hose 
from  touching  the  bottom,  in  which  case  it  would  draw  precipitated 
silver;  or  the  clear  solution  can  be  removed  also  by  more  convenient 
syphon,  shown  by  d,  which  is  made  of  a  lead  pipe.  By  lowering 
the  same,  the  clear  solution  commences  to  flow  through  the  outside 
end,  as  soon  as  it  sinks  below  the  level  of  the  solution. 

The  solution  from  the  precipitating  vat  runs  through  the  gutter,  6, 
figure  15,  into  a  large  vat,  from  which  it  is  pumped  up  into  the  solu- 
tion vat,  standing  above  leaching  vats.  The  hyposulphite  of  lime  is 
thus  constantly  circulating  from  the  uppermost  solution  vat  through 
the  leaching  vat  into  the  precipitating  tubs,  and  from  thence  into  the 


116  LIXIVIATION. 

lowest  receiving  vat,  again  to  be  pumped  up  into  the  solution  vat. 
For  this  purpose  wooden  pumps  answer  very  well. 

Treatment  of  the  Precipitated  Silver. 

After  the  precipitated  silver  has  accumulated  to  10  or  12  inches 
deep,  and  the  clear  solution  of  hyposulphite  of  lime  has  been  removed, 
the  precipitate  can  be  taken  out  by  the  hole  at  the  bottom  of  the  vat 
provided  with  a  large  brass  cock,  as  shown  in  Figure  15. 

Each  precipitating  tub  has  a  filter  of  cotton  cloth,  h,  tacked  on  a 
wooden  frame  about  two  feet  wide  and  from  three  to  four  feet  long, 
or  differently  shaped  niters  like  those  used  for  amalgam.  Into  these 
filters  the  precipitate  is  allowed  to  flow  until  almost  full,  and  after  the 
solution  has  filtered  through  and  the  precipitate  settled,  more  of  the 
silver  is  allowed  to  flow  in,  this  being  repeated  several  times  till  the 
filter  remains  well  charged.  After  this,  clear  water  is  admitted  till  all 
hyposulphite  of  lime  is  removed  and  no  taste  is  observed  in  the  filter- 
ing water. 

After  several  hours  the  black  precipitate  must  be  pressed  to  get 
out  the  remaining  water.  A  screw  press  answers  well  for  this  purpose, 
but  the  precipitate  should  be  wrapped  up  in  a  cloth  and  then  pressed. 
The  black  silver  cakes  are  then  taken  out  and  dried  in  a  warm  room, 
or  in  a  drying  oven.  For  the  purpose  of  burning  off  the  sulphur,  the 
dried  sulphide  is  introduced  into  a  muffle  or  other  calcining  furnace, 
and  heated  till  the  sulphur  commences  to  burn  with  its  known  blue 
flame.  When  this  disappears  the  heating  must  continue  at  a  dark 
red  heat  for  several  hours.  By  this  operation  the  cakes  are  reduced 
almost  entirely  to  metallic  silver,  generally  covered  with  threads  of 
silver;  sometimes  an  intense  green  color  is  assumed  by  pieces  remain- 
ing in  the  furnaces  over  night. 

The  burned  cakes  are  now  prepared  for  smelting  in  crucibles.  They 
are  placed  in  black  lead  crucibles,  according  to  the  size,  up  to  three 


LIXIVIATION.  117 

hundred  pounds,  and  fused.  All  the  sulphur  was  not  driven  out  by 
the  preceding  operation.  The  remaining  part  must  be  removed  by 
placing  metallic  iron  in  the  fused  metal ;  thereby  iron  matte  is  formed 
which  rises  to  the  surface  and  is  skimmed  off.  The  surface  of  the 
silver  is  then  cleaned  by  adding  some  bone  ash  and  borax,  or  borax 
alone,  which  is  also  skimmed  off  and  the  silver  dipped  out  or  poured 
out  into  moulds.  According  to  the  careful  treatment  in  the  roasting 
process,  and  the  nature  of  the  ore,  the  silver  will  be  from  800  to  950 
fine. 

Mr.  O.  Hofmann,  in  want  of  sulphur  for  the  production  of  sulphide 
of  calcium,  used  to  calcine  the  dried  sulphide  of  silver  in  iron  retorts. 
In  this  way  he  obtained  a  large  proportion  of  sulphur  as  a  fine  subli- 
mate. This  could  be  done  also  in  a  proper  muffle  furnace,  so  arranged 
that,  after  all  obtainable  sulphur  had  sublimated  in  a  receiver,  this 
could  be  removed,  and  the  calcination  continued  under  access  of  air. 
At  Melrose,  0.  Hofmann  tried  to  dry  the  silver  precipitate  by  a  centri- 
fugal machine,  with  excellent  result,  in  less  than  one  minute. 

Applicability  of  the  Lixiviation  Process. 

All  silver  ores  which  allow  a  good  chlorination  in  the  furnace,  and 
which  permit  filtration,  are  suitable  for  the  lixiviation  process.  The 
great  advantage  of  this  process  is  cheapness.  Roasting,  of  course,  is 
indispensable,  except  with  chloride  ores;  but  neither  pans  and  the  re- 
quired power,  nor  quicksilver,  are  used,  and  for  this  reason  less  capital 
is  necesary  to  put  up  reduction  works.  All  the  cupreous  silver  ores 
of  Cerro  Gordo,  Yellow  Pine,  Montgomery,  and  of  the  other  new 
silver  districts,  can  be  treated  to  great  advantage  by  the  solving  pro- 
cess. But  not  only  the  dispensing  of  pans,  power,  and  whatever 
belongs  to  them,  deserve  consideration.  In  using  pans,  there  is 
gearing,  belting  and  other  parts  exposed  to  breakage ;  the  pans  wear 
out  in  time,  especially  with  base  roasted  ore.  The  cost  of  quicksilver 


118  LlXIVIATION. 

itself  is  not  great,  provided  it  has  not  to  be  transported  too  far ;  but 
this  process  is  important  in  localities  where  lime  and  sulphur  can  be 
got  in  the  neighborhood,  while  the  transportation  of  quicksilver  is 
troublesome  and  costly.  The  process,  although  a  little  more  compli- 
cated than  the  pan  amalgamation,  and  requiring  more  time,  is  never- 
theless simple  and  easily  learned. 

Ores  containing  much  clay  and  lime,  or  talk,  filter  very  slowly,  some- 
times so  slowly  that  pan  amalgamation  must  be  preferred.  The  remedy 
in  such  case  is  found  in  separation  of  the  mud  or  slime  from  the  coarser 
sandy  part.  The  crushing,  therefore,  must  be  changed  to  wet  crush- 
ing, and  the  separation  must  be  effected  either  through  pointed  boxes 
or  some  other  arrangement,  where  the  flow  of  water  is  swift  enough 
to  carry  the  fine  stuff  further  than  the  sandy  part.  This  latter,  for 
the  purpose  of  roasting,  must  be  dried.  Generally,  it  is  charged  wet 
in  a  roasting  furnace  with  two  or  three  hearths.  In  this  case,  the 
salt  is  added  when  the  ore  is  in  the  last  hearth.  But  then  comes  the 
inconvenience  of  handling  the  slimes.  In  Mexico,  this  part  can  be 
subjected  to  the  patio  amalgamation,  provided  the  silver  comes  out  by 
this  process,  and  in  the  United  States  the  slime,  on  account  of  its 
extreme  fineness,  may  admit  a  direct  pan  amalgamation ;  but  in  either 
case  a  double  manipulation  is  very  inconvenient.  The  proposal  to 
treat  such  ore  without  separation  in  an  agitator  with  a  filtering  bot- 
tom, may  perhaps  answer  in  some  instances  ;  but  this  point  must  be 
decided  by  experiments. 

Ore  that  permits  filtration,  but  too  slowly,  can  be  leached  much 
quicker  with  hot  solutions,  to  begin  with  hot  water ;  but  the  hot  water 
in  the  beginning  dissolves  more  chloride  of  silver  than  cold  water. 

Lixiviation  with  aid  of  suction  is  a  great  advantage  where  it  can 
be  arranged.  The  leaching  boxes  must  be  located  ten  to  twelve  feet 
above  the  precipitating  vats.  The  arrangement  is  represented  by 
Figure  16. 


LIXIVIATION. 
Fig.  16. 


119 


The  hose,  c,  is  inserted  at  the  lowest  point  of  the  vat  below  the 
false  bottom ;  then  there  is  a  glass  pipe,  d,  reaching  outside  near  the 
rim  of  the  vat.  The  hose,  before  commencing,  is  likewise  lifted  as 
high  as  the  glass  tube,  a  represents  the  first  staves  on  the  bottom  of 
the  vat,  and  b  the  second  row  which  is  covered  with  the  cloth.  After 
the  vat  is  filled  with  ore  and  the  water  admitted,  the  air  below  the 
false  bottom  escapes  through  both  pipes ;  but  after  a  while,  when  the 
water  rises  and  covers  the  mouth  of  the  hose,  the  air  from  between 
the  upper  staves  goes  through  the  glass  tube  and  is  followed  by  the 
water.  When  this  reaches  the  water  level  inside  the  tank,  the  tube 
is  shut  up  by  a  tight  stopper  and  the  hose  lowered  down  to  the  pre- 
cipitating vats.  The  lixiviation  is  much  quicker  than  without  suc- 
tion, but  if  the  hose  should  be  so  wide  that  more  water  escapes  than 
can  follow,  there  will  be,  of  course,  no  suction. 


Hyposulphite  of  Soda. 

The  hyposulphite  of  soda  is  a  cheap  commercial  article  of  white 
transparent  crystals  (5  to  6  cts.  per  Ib.).  It  is  made  in  different  ways. 
Four  parts  of  glauber  salt  (calcined)  are  mixed  with  1  to  1 J  parts  of 
charcoal  or  coal  dust  moistened  a  little,  and  introduced  into  a  large 
crucible  or  in  an  iron  vessel,  and  then  exposed  to  a  red  heat  from  six 


120  LIXIVIATION. 

to  ten  hours.  After  this,  it  is  pulverized,  moistened  again,  brought  in 
contact  with  sulphurous  acid  in  thin  layers,  lixiviated  with  water,  and 
the  water  evaporated  sufficiently  to  make  the  salt  crystalize.  A 
cheaper  method  is  to  use,  for  this  purpose,  lime  which  has  been  used 
in  gas  factories  for  purifying  gas.  This  lime,  if  exposed  to  the  action 
of  the  air  for  some  time,  is  changed  into  hyposulphite  of  lime,  and  is 
easily  decomposed  by  carbonate  of  soda.  But  the  cheapest  way  for 
metallurgical  purposes  is  to  buy  this  article,  as  several  hundred  pounds 
is  all  that  is  needed  to  commence  with.  Afterwards,  if  carefully 
handled,  by  the  use  of  sulphide  of  calcium,  the  quantity  of  the  hypo- 
sulphite increases,  and  as  there  is  always  some  waste  of  soda  replaced 
by  lime,  the  whole  liquid  is  changed  by  degrees  into  hyposulphite  of 
lime.  If  there  is  a  small  amount  of  gold  in  the  ore,  the  hyposulphite 
of  soda  does  not  dissolve  the  gold,  which,  after  roasting,  is  converted 
into  a  combination  with  chloride  of  sodium  ;  but  the  hyposulphite 
of  lime  does  dissolve  it.  In  this  case,  if  there  is  gold  in  the  ore,  it 
may  be  of  interest  to  commence  directly  with  the  lime  solution.  The 

Hyposulphite  of  Lime 

is  best  prepared  by  taking  three  parts  of  lime  to  one  and  a  half  parts 
of  flour  of  sulphur  and  boiling  it  either  in  an  iron  vessel  or  in  a  wooden 
tub,  making  first  sulphide  of  calcium,  by  introducing  steam  through 
an  iron  pipe,  to  which  a  circular  shape  is  given  at  the  bottom  with 
several  little  holes  for  the  exit  of  the  steam.  After  the  fresh  burned 
lime  is  charged,  the  water  is  admitted  sufficiently  to  cover  the  lime 
several  inches.  The  boiling  commences  immediatelyjand  the  sulphur 
can  be  charged  without  delay.  When  the  lime,  during  the  stirring, 
seems  to  be  in  a  soft,  pulpy  condition,  it  probably  requires  more  water, 
which  must  be  added,  then  sufficient  steam  can  be  admitted  to  keep 
the  mass  boiling,  after  which  it  will  soon  begin  to  assume  a  yellow 
color ;  water,  if  necessary,  must  be  added  to  keep  the  lime  sufficiently 
liquid.  The  boiling  must  continue  from  four  to  six  hours.  If  the 


LIXIVIATION.  121 

lime  is  of  good  quality,  one  and  a  half  pounds  of  sulphur  can  be  taken 
to  three  of  lime.  Polysulphide  of  calcium  is  formed,  mixed  with  hy- 
posulphite of  lime  and  with  bisulphide  of  calcium,  and  is  now  in  the 
condition  used  to  precipitate  the  silver.  The  boiling  after  four  or  six 
hours  is  stopped.  After  ten  or  twelve  hours,  the  solution  appears 
clear  above  the  undissolved  lime  on  the  bottom. 

It  will  soon  be  found  out  by  practice  how  much  water  is  required 
to  obtain  a  solution  of  from  8  to  10  degrees  Baume',  and  should  not 
be  less  than  6°  if  intended  for  precipitation ;  but  to  convert  it  into 
hyposulphite  of  lime,  3°  to  4°  will  answer. 

The  clear  solution  is  then  drawn  off,  either  with  a  hose  used  as  a 
syphon,  or  it  can  be  discharged  through  several  holes  in  the  tub,  one 
above  the  other,  provided  with  short  pipes.  After  all  the  clear  liquid 
runs  out,  the  tub  is  filled  with  water  as  high  as  it  was  before  and 
boiled  again.  If  this  second  solution  should  be  too  weak  for  precipi- 
tation, it  can  be  used  for  dilution  of  another  fresh  charge  of  lime  after 
the  sulphur  has  been  introduced.  The  remaining  lime  in  the  vat  con- 
tains still  some  hyposulphite  which  may  be  obtained  by  filtration,  but 
it  niters  very  slowly. 

The  clear  yellow  liquid  is  now  operated  upon  with  sulphurous  acid 
gas  introduced  by  an  iron  pipe,  and  continued  till  the  yellow  color 
disappears,  and  a  few  drops  of  it  do  not  make  the  slightest  precipita- 
tion in  a  diluted  solution  of  chloride  of  silver  in  hyposulphite  of  soda. 
It  is  now  ready  to  be  used  as  hyposulphite  of  lime. 

The  cheapest  way  to  produce  sulphurous  acid  under  such  pressure 
as  to  conduct  it  into  a  liquid  through  iron  pipes,  is  to  use  sulphuric 
acid  with  charcoal.  For  this  purpose  an  iron  retort  is  charged  with 
small  charcoal  (which  would  pass  through  a  sieve  of  four  holes  to  the 
running  inch)  to  the  half  of  its  capacity;  then  as  much  sulphuric  acid  in- 
troduced as  to  form  a  pulp.  After  this  the  retort,  surrounded  by  sand, 
is  heated  until  the  gas  commences  to  escape  through  the  pipe ;  the  heat 
be  kept  uniform,  It  is  not  necessary  here  to  clean  the  gas. 


122  LIXIVIATION. 

From  time  to  time  sulphuric  acid  may  be  introduced  into  the  retort, 
then  again  charcoal.  The  sulphuric  acid  is  most  suitable  of  1.82 
specific  gravity. 

Sulphide  of  Calcium, 

the  manufacture  of  which  was  described,  together  with  the  hyposul- 
phite, gives  a  much  better  precipitation  than  the  same  salt  of  sodium. 
It  precipitates  quicker,  and  filters  better ;  but  the  precipitated  silver, 
if  not  washed  with  water  before  being  pressed,  retains  the  calcium, 
and  is  more  difiicult  to  melt.  The  sulphide  can  be  kept  in  an  open 
vessel  for  a  long  time,  but  the  hyposulphite  decomposes  soon  if  not  in 
use.  Boiling  the  lime  with  sulphur  in  order  to  make  sulphide  of  cal- 
cium, the  quality  of  lime  mnst  be  considered. 

The  sulphide  is  formed  only  from  caustic  lime,  consequently  more 
is  obtained  from  fresh  burned  lime.  Of  this  a  certain  quantity  is 
charged  into  an  iron  kettle,  water  added,  and  then  the  pulverized 
sulphur.  The  proportion  of  sulphur  and  lime  depends  on  the  quality 
of  the  latter.  The  purest  quality  of  lime  from  Santa  Cruz,  California, 
for  instance,  takes  one  pound  of  sulphur  to  1.33  of  lime.  Of  poorer 
qualities  of  lime  it  is  better  to  take  three  pounds  to  one  of  sulphur  and 
about  ten  parts  of  water.  It  is  kept  boiling  for  four  or  six  hours, 
stirred  with  a  wooden  stirrer  from  time  to  time,  and  then  allowed  to 
cool  and  clear,  and  drawn  off  into  wooden  or  iron  tubs.  In  the  residue, 
which  may  be  drawn  into  a  filter-box,  there  will  be  found  more  or  less 
reddish-yellow  crystals.  These  crystals  are  bisulphide  of  lime,  and 
would  serve  likewise  for  precipitation,  but  they  require  about  400 
parts  of  water  to  be  dissolved,  and  in  this  condition  the  liquid  is  too 
weak  to  be  of  any  use.  The  shorter  the  boiling  time  is,  or  if  in  pro- 
portion too  little  sulphur  is  taken,  so  much  more  of  these  crystals  will 
be  formed. 

Working  Auriferous  Silver  Ores. 

Ottakw  HofuKmn'a  Patent  Process.  If  rich  auriferous  silver  ores, 
in  which  the  percentage  of  gold  is  high,  almost  equal  to  that  of  silver, 


LIXIVIATION.  123 

should  be  subjected  to  a  chloridizing  roasting,  then  impregnated  with 
chlorine  gas,  leached  with  water,  for  the  purpose  of  extracting  the 
gold,  and  finally  leached  with  hyposulphite  of  lime  for  the  silver,  it 
would  in  this  case,  although  a  high  percentage  of  silver  might  be  ex- 
tracted, result  in  a  yield  of  gold  that  would  hardly  amount  to  much 
more  than  about  50  per  cent.,  more  or  less.  The  reason  is  not  easily 
explained ;  the  gold  may  be  influenced  somehow  by  the  base  metal 
chlorides  during  the  roasting,  which  prevent  the  gold  from  being  at- 
tacked by  the  chlorine  gas. 

On  the  other  hand,  if  the  base  metal  chlorides  and  the  chloride  of 
silver  are  extracted  previous  to  the  impregnation  with  chlorine,  both 
metals — silver  and  gold — can  be  got  out  very  close.  On  this  fact  is 
based  O.  Hofmann's  gold  and  silver  chlorination  process,  by  which  he 
successfully  treated  the  gold  and  silver-bearing  sulphurets  of  the  Colo- 
rado No.  2,  G.  &  S.  M.  Co.,  at  Monitor,  Alpine  county,  Cal. 

The  ore  of  Colorado  No.  2,  better  known  by  the  name  "Tarshish 
mine,"  as  it  comes  from  the  mine  is  poor,  assaying  in  average  from 
seven  to  nine  dollars  per  ton,  but  the  gangue,  being  decomposed  feld- 
spar, causes  the  ore  to  be  soft  and  easily  crushed,  and  is  by  means  of 
Frue's  concentrators  very  highly  concentrated.  The  concentrated  sul- 
phurets consist  of  iron  pyrites,  silver  glance,  silver  copper  glance, 
fahlore,  ruby  silver,  zincblende  and  galena.  The  value  of  the  concen- 
tration varies  a  great  deal,  ranging  from  $250  to  $1,700  per  ton,  with 
over  40  per  cent,  of  the  value  in  gold,  which  is  not  free  in  the  ore. 
The  average  value  of  five  months'  run  proved  to  be  $564  per  ton — <. 
$333.19  in  silver  and  $230.84  in  gold. 

The  operations  of  the  process  are  as  follows  : 

1.  The  Boasting.  The  concentrated  sulphurets  are  subjected  to  a 
chloridizing  roasting  in  a  reverberatory  furnace.  These  furnaces, 
although  old-fashioned  st;rring  furnaces,  are  considered  by  Hofmann 
preferable  for  this  class  of  ores  to  any  mechanical  furnace  in  use. 


124  LIXIVIATIOX. 

especially  to  the  continuous  discharging  ores.  Those  concentrated 
sulphurets  require  a  very  perfect  roasting,  and  the  furnaces,  with  con- 
tinuous discharge,  do  not  give  time  enough  for  very  high  sulphureted 
ore  to  become  thoroughly  desulphurized.  Such  high-grade  ores  re- 
quire close  attention,  and  the  process  must  be  under  perfect  control 
of  the  roaster.  However,  it  does  not  take  near  as  much  time  as  re- 
quired by  Plattner's  gold  chlorination. 

After  the  ore  has  been  roasted,  it  is  spread  on  the  cooling  floor  and 
sifted,  when  cold,  through  a  sieve  of  ten  to  fourteen  meshes  to  the 
running  inch.  The  sulphurets  are  heavy  enough  after  roasting  to 
make  very  little  dust  during  the  sifting,  so  that  the  inconvenient 
moistening  can  be  obviated.  The  lumps  are  saved  till  a  larger  amount 
accumulates ;  they  are  then  pulverized  in  a  dry  battery  and  slightly 
roasted. 

2.  Leaching  of  Base  Metals.  The  roasted  and  sifted  ore  is  charged 
into  tanks  with  filter  bottom,  in  quantities  from  two  and  a  half  to 
three  tons,  and  leached  with  water  to  extract  all  soluble  base  chlo- 
rides. The  watery  if  saturated  with  those  base  chlorides  and  some 
salt,  of  which  generally  some  is  left  in  the  ore  undecomposed  after 
roasting,  acts  on  the  chloride  of  silver  like  a  concentrated  brine,  dis- 
solving the  same.  To  prevent  the  escape  of  this  dissolved  part  of 
silver,  Hofmann  does  not  admit  the  water  from  above  the  ore,  as  is 
usually  done,  but  from  below  the  filter-bottom,  which,  by  means  of  a 
slight  pressure,  is  forced  to  ascend  through  the  ore  to  the  top  of  the 
vat.  In  this  way  the  concentrated  solution  accumulates  above  the 
ore,  and  in  diluting  the  same  by  a  stream  of  water  and  permitting  the 
solution  to  flow  out  through  the  filter-bottom,  the  chloride  of  silver  is 
precipitated  on  and  through  the  ore,  which  is  then  extracted  with  the 
balance  of  the  silver.  This  operation  effects  the  fineness  of  the  bul- 
lion somewhat,  if  there  is  a  considerable  amount  of  lead  in  the  ore, 
but  not  materially.  The  bullion  of  Colorado  No,  2  has  by  this  pro- 
cess a  fineness  of  957, 


LIXIVIATION.  125 

3.  Leaching  of  the  Silver.     This  is  done  as  usually  by  leaching  the 
ore  with  a  solution  of  hyposulphite  of  lime,  and  precipitating  the  silver 
with  polysulphide  of  calcium.     The   hyposulphite  dissolves  more   or 
less  gold,  so  that  the  bullion  of  Colorado  No.  2  contains  from  two  to 
ten  thousandths  of  gold. 

4.  Second  Leaching  with  Water.     After  the  silver  has  been  ex- 
tracted, the  solution  of  the  hyposulphite  is  allowed  to  run  out  till  it 
disappears  under  the  surface  of  the  ore,  when  clear  water  is  introduced 
again,  in  order  to  displace  all  solution.     The  desilvered   ore   is   then 
removed  from  the  tank  to  a  dry   kiln,   where  it  is  left  for  a  time  till 
the  surplus  water  has  evaporated.     After  this,  it  is  charged  back  into 
the  tank  still  moist.     This   second   handling   and   drying  cannot  be 
avoided,  as  the  ore  after  leaching  is  too  wet  to  permit  of  a  free  passage 
of  the  chlorine  gas,  but  if  the  works  are  arranged  properly,  this  partial 
drying  causes  neither  much  delay  nor  much  expense. 

5.  Gold  Extraction.     The  gold  in  the  ore  is  now  in  metallic  con- 
dition, and  very  bright  and  clean,  permitting  a  very  close  extraction. 
The  rim  of  the  tank  is  provided  with  a  groove,  which  is  open  towards 
the  inside,  two  and  a  half  inches  deep  and  one  and  a  half  inches  wide. 
In  this  groove  fits  the  cover  of  the  tank,  leaving,  however,  a  play  of 
one-eighth    of  an  inch  around  the  circumference.     The  cover  being 
made  of  one-inch  boards,  the  staves  of  the  tank  will  project  one  and  a 
half  inches  above  the  cover.     This  arrangement  serves  in  two  ways — 
first,  it  facilitates  the  operation  to  make  the  cover  air-tight  with  clay, 
and  second,  it  enables  the  chlorination  to  keep  a  sheet  of  water  one 
inch  thick  on  the  top   of   the  cover,  thus  making  the  cover  perfectly 
air-tight   and  preventing   the   escape  of  gas  into  the  working-room 
during  the  time   of   charging  the  tank  with  water  for  the  purpose  of 
extracting  the  gold.     The  cover   is,    furthermore,  provided  with  two 
pieces  of  one  and  a  fourth-inch  gas  pipe,  six  inches  long,  and  a  square 
opening  six  by  six  inches.     During  the  time  of  the  impregnation  of 


126  LIXIVIATION. 

the  ore  with  gas,  these  pipes  are  closed  with  balls  of  clay.  As  soon 
as  the  charge  is  ready  for  the  extraction  of  the  gold,  these  balls  are 
removed  and  one  of  the  pipes  is  connected  with  the  hose  of  the  water- 
tank,  while  the  other,  by  means  of  a  hose,  either  with  another  tank 
already  prepared  for  chloridizing,  or  with  the  ash-pit  of  the  roasting 
furnace.  This  is  done  to  utilize  the  surplus  of  chlorine  gas,  and  to 
protect  the  workmen  from  the  very  injurious  effect  of  the  same.  Care 
must  be  taken  to  have  sacks  placed  on  the  top  of  the  ore  right  under 
the  water  pipe,  and  kept  in  place  by  two  bricks  to  prevent  the  stream 
of  water  working  into  the  ore.  The  square  opening  serves  for  exam- 
ining the  progress  of  the  gas  in  the  ore,  and  can  be  closed  air-tight  by 
a  good  fitting  cover  and  clay. 

The  chlorine  gas  is  generated  in  a  leaden  gas  generator,  which  is 
not  heated,  as  usual,  by  direct  application  of  fire,  but  by  steam.  For 
this  purpose  the  generator  is  placed  into  a  tight-fitting  box,  leaving  a 
space  of  two  inches  around  the  side  and  bottom  for  the  circulation  of 
steam.  The  rim  and  cover  are  kept  outside  the  box.  The  steam 
enters  on  one  side  through  a  half-inch  pipe,  while  the  other  side  of 
the  box  is  provided  with  a  one-inch  exhaust  pipe  and  an  outlet  for  the 
water. 

Wherever  steam  can  be  had  in  a  chlorination  work,  this  arrange- 
ment will  prove  very  convenient  and  useful.  The  operator  has  the 
temperature  entirely  under  his  control.  The  least  turn  on  the  valve 
increases  or  decreases  the  heat,  and,  of  course,  regulates  the  genera- 
tion of  gas.  The  discharge  pipe  of  the  generator  projects  a  short  way 
out  of  the  box.  On  this  pipe  is  fastened  a  piece  of  hose  about  two 
feet  long,  which  can  be  closed  with  a  thumbscrew  clamp.  The  hose 
lies  in  a  covered  trough,  which  leads  outside  of  the  gas-house.  In 
discharging  the  generator  a  small  stream  of  water  is  permitted  to  flow 
through  the  funnel  into  the  generator,  by  whiclf  the  gas  is  forced 
through  its  usual  outlet  into  cue  of  the  tanks,  When  the  generator 


LIXIVIATION.  127 

is  filled,  the  stirrer  is  set  in  motion  and  the  thumbscrew  on  the  hose 
loosened.  In  this  way  the  generator  can  be  discharged  without  mo- 
lesting or  injuring  the  men.  The  cover  of  the  generator  is  closed 
tightly  with  clay  and  only  removed  in  case  of  repair. 

The  gas  is  conducted  through  a  leaden  pipe,  intersected  with  rub- 
ber hose  alongside  and  in  front  of  the  tanks,  almost  in  the  same 
height  as  the  top  of  them.  By  means  of  a  T  and  rubber  hose,  each 
tank  is  connected  with  the  main  pipe,  and  can  be  disconnected  by  the 
use  of  a  thumb-screw  clamp.  The  pipe  through  which  the  gas  enters 
the  tank  is  independent  from  that  one  through  which  the  solution 
discharges.  It  is  placed  higher  and  as  close  to  the  false  bottom  as 
possible. 

After  the  gas  has  been  the  proper  time  in  contact  with  the  ore,  the 
gold  is  extracted  by  water,  and  precipitated  in  the  usual  way  with 
sulphate  of  iron.  The  gold  obtained  is  of  extreme  fineness,  varying 
from  970  to  987  thousandths. 

In  treating  very  rich  ores,  containing  say  $700  to  $800  gold  per  ton, 
the  solution  carrying  out  the  gold  is  of  a  very  lustrous  yellow  color, 
and  the  precipitated  gold  accumulates  on  the  bottom  is  spongy  lumps  of 
great  specific  gravity,  some  of  them  showing  scales  of  bright  gold, 
which,  under  the  microscope,  might  prove  to  be  crystallized  gold. 
There  is  but  very  little  more  time  used  in  leaching  rich  gold  ore  than 
poor. 

If  the  ore  is  copperous,  considerable  copper  will  be  carried  out 
with  the  gold  solution,  coloring  the  same  green.  In  order  to  save 
the  copper,  the  solution,  after  the  gold  has  been  precipitated  and 
settled,  is  decanted  into  the  copper  tanks.  But  before  doing  this  it 
is  advisable  to  draw  the  solution  first  into  a  second  gold  tank,  in  order 
to  catch  the  gold  which  should  accidentally  be  carried  off  with  the 
stream,  and  to  leave  it  there  for  some  time  to  let  the  gold  settle 
again. 


128  LIXIVIATION. 

RESULTS.— To  ascertain  the  working  result  of  this  process  the 
concentrated  raw  sulphurets  of  the  Colorado  No.  2  obtained  each  day 
had  carefully  been  weighed  and  assayed  during  a  period  of  five 
months.  The  average  value  of  these  concentrations,  as  mentioned 
above,  had  been  during  said  five  months  $564  per  ton,  with  over  40 
per  cent  in  gold. 

The  total  value  of  the  bullion  shipped  at  the  end  of  this  period, 
compared  with  the  net  value  of  the  raw  sulphurets  worked  during 
the  same  time,  showed  the  actual  working  result  to  be,  silver  96  per 
cent.,  gold  95  per  cent. 

Patera   Process. 

The  most  delicate  operation  in  Patera's  process  is  the  preparation 
of  the  ore  by  roasting.  The  chloride  of  silver,  formed  during  the 
roasting  is  dissolved  by  a  cold  solution  of  hyposulphite  of  soda, 
after  all  soluble  base  metals  have  been  first  leached  out  with  hot 
water.  Two  parts  of  the  hyposulphite  of  soda  dissolve  one  part  of 
chloride  of  silver,  forming  a  soluble  double  salt.  The  tubs  in  which 
the  ore  is  lixiviated  with  the  hyposulphite  of  soda  are  small,  receiving 
only  200  pounds  of  roasted  ore.  The  extraction  of  silver  is  performed 
in  the  same  way  as  described  (page  104.) 

Patera's  process,  to  extract  silver,  cobalt  and  nickel,  in  the  wet 
way,  after  an  oxidizing  roasting  of  six  hours,  is  as  follows:  The 
roasted  ore  is  placed  in  wooden  vats  with  an  addition  of  diluted 
sulphuric  acid.  Steam  is  introduced  into  the  pulp  and  heated  up  to 
100°  F.  The  greatest  part  of  the  nickel  and  cobalt  are  now  dissolved, 
while  the  silver  remains  unattacked.  After  the  nickel  and  cobalt 
solution  is  drawn  off  or  leached,  diluted  nitric  acid  is  poured  on  the 
ore  and  again  heated  by  steam,  till  the  creation  of  red  fumes  ceases. 
This  solution  is  also  drawn  off,  the  ore  washed  with  hot  water,  and 
the  silver  precipitated  by  salt.  Besides  the  silver  there  is  also  cobalt, 


LIXIVIATION.  129 

nickel,  some  iron  and  arsenic,  in  this  second  solution.  To  make  the 
precipitated  chloride  of  silver  fall  quicker  to  the  bottom,  the  liquid  is 
brought  into  motion,  and  then  after  the  solution  appears  clear,  it  is 
removed,  by  means  of  a  syphon,  and  on  account  of  some  floating  silver, 
permitted  to  stay  quiet  for  twelve  hours. 

The  chloride  of  silver,  with  diluted  sulphuric  acid,  is  conveyed  into 
a  vat,  and  precipitated  in  metallic  condition  by  old  clean  iron.  The 
silver  is  washed,  dried,  and  melted  into  bars, 

The  solution  containing  nickel  and  cobalt  is  mixed  with  chloride  of 
iron,  and  neutralized  by  addition  of  pulverized  lime  rock.  Arsenate 
of  iron  and  some  iron  oxide  are  thus  precipitated;  and  the  liquid,  now 
purified,  is  condensed  by  evaporation.  From  this  condensed  liquid 
the  cobalt  is  preciptated,  by  chlorate  of  lime,  the  solution  separated, 
and  the  nickel  precipitated  by  fresh  burned  lime.  The  nickel  oxide 
is  filtered,  pressed,  dried  and  calcined.  After  this  it  is  ground  fine, 
and  then  mixed  with  five  per  cent,  of  rye  flour,  and  sufficient  syrup 
to  form  a  stiff  dough,  which  is  cut  into  cube  pieces,  dried,  placed  with 
charcoal  powder  into  a  crucible,  and  exposed  to  an  intense  white  heat. 
The  nickel  oxide  is  reduced  to  metallic  nickel ;  the  flour  and  syrup  is 
burned  off,  and  the  nickel  retains  the  cube  shape,  and  appears  brightly 
polished,  after  being  kept  revolving  in  a  barrel  with  water  for  some 
hours. 

Kiss  Process. 

This  process  extracts  silver,  and,  to  some  extent,  the  gold.  Roast- 
ing the  ore,  a  part  of  the  gold  is  transformed  into  such  a  state  as  to- 
render  it  insoluble  in  water.  After  roasting,  the  ore  is  placed  in 
filtering  tubs  and  washed  with  water  to  remove  the  base  metals. 
After  this,  a  solution  of  hyposulphite  of  lime  is  conveyed  on  the  ore, 
by  which  the  gold  and  silver  chlorides  are  dissolved  and  carried  off 
into  precipitating  tubs.  As  soon  as  the  sulphide  of  calcium  is  intro- 
duced, the  gold  and  silver  are  precipitated  as  sulphides.  The  precipi- 


130  LIXIVIATION. 

tation  of  both  metals  in  a  metallic  condition  is  not  admissible,  for  the 
reason  that  the  hyposulphite  of  lime  is  decomposed  if  metallic  copper 
is  employed  for  precipitation.  The  results  of  Kiss's  methods,  prac- 
ticed in  Hungary,  were  not  altogether  satisfactory  with  concentrated 
sulphurets,  but  gave  a  good  result  with  auriferous  silver  ores.  It 
depends  a  great  deal  on  the  roasting  whether  more  or  less  gold  is 
extracted  by  the  Kiss  process.  If  a  high  heat  is  applied,  then  the 
greatest  part  of  the  gold  is  set  free  in  metallic  condition,  and  in  this 
case  very  little  gold  can  be  dissolved  by  the  hyposulphite  of  lime, 
because  the  metallic  gold  remains  untouched,  and  only  that  part  of  it 
which  during  a  slow  cooling  after  the  discharge,  while  the  chlorine 
still  escapes,  happens  to  change  into  subchloride  of  gold,  would  be 
dissolved.  If,  however,  the  roasting  is  conducted  at  a  dark  red  heat, 
just  enough  to  chloridize  the  silver,  then  a  larger  part  of  the  gold  will 
combine  as  an  oxide  with  soda  and  chloride  of  sodium,  besides  form- 
ing subchloride  of  gold,  both  combinations  being  insoluble  in  water, 
but  dissolve  in  hyposulphite  of  lime. 

Patera  and  Roeszner  Processes. 

The  object  of  this  process  is,  like  that  of  the  preceding,  the  extrac- 
tion of  silver  and  gold  together.  The  ore  is  first  subjected  to  a  chlo- 
ridizing  roasting,  "By  which  the  silver  is  converted  into  a  chloride, 
while  the  gold  remains  mostly  in  metallic  condition.  The  leaching 
liquid  is  prepared  by  conveying  chlorine  gas'  through  a  cold  concen- 
trated solution  of  salt  to  saturation.  This  chloridized  solution  dis- 
solves silver,  gold  and  copper  at  the  same  time.  The  roasted  ore  is 
charged  into  tubs  with  false  bottoms,  and  the  cold  solution  of  salt  and 
chlorine  introduced.  Silver  ores  treated  after  this  method  in  Hungary 
gave  98.94  per  cent,  of  the  silver,  all  the  copper,  and  nearly  all  the 
gold.  An  experiment  on  five  tons  of  ore  gave  a  clear  profit  of  seventy- 
five  florins,  compared  with  the  amalgamation. 


LIXIVIATION.  131 

Roeszner  roasts  the  Ore  with  salt,  extracts  a  part  of  the  silver  by 
Augustin's  method  with  a  hot  solution  of  salt,  and  treats  the  residue 
alternatel}'-  with  a  solution  of  salt  and  chlorine,  and  with  a  hot 
concentrated  salt  solution  for  the  extraction  of  gold  and  the  remainder 
of  the  silver. 

Augustin    Process. 

This  process  is  not  in  use  at  present  for  silver  ores,  but  for  products 
of  smelting.  By  this  method  the  chloride  of'  silver,  which  is  formed 
by  way  of  roasting,  is  dissolved  in  a  hot  solution  of  salt,  and  precipi- 
tated by  metallic  copper.  One  part  of  chloride  of  silver  requires 
sixty-eighfc  parts  of  salt  to  be  dissolved. 

Extraction  of  the  Silver  from  Copper  Matte  and  Black  Copper. 
The  principal  aim  with  these  materials  is  the  oxidation  of  the  copper 
as  perfectly  as  possible,  and  then  the  chlorination  of  the  silver.  There 
are  wooden  leaching  tubs  of  a  small  size — two  feet  eight  inches  in 
diameter,  and  nearly  four  feet  high — fixed  on  wheels  and  arranged  in 
one  row.  Into  these  tubs,  which  have  false  bottoms,  the  roasted  stuff 
is  introduced — about  800  pounds  in  each.  Ores  containing  different 
kinds  of  earths  cannot  be  lixiviated  at  a  depth  of  over  three  feet; 
the  metal  oxides,  however,  allow  the  water  to  pass  freely.  This  is 
also  the  case  with  roasted,  concentrated,  or  pure  sulphurets.  Hot 
solution  of  salt  is  now  allowed  to  flow  through  a  trough  in  each  tub. 
The  salt  penetrates  the  powder,  dissolves  the  chloride  of  silver,  and 
carries  it  through  the  filter  at  the  bottom  of  the  tubs,  flows  off  to  a 
reservoir,  and  from  here,  after  the  particles  which  may  escape  through 
the  filter  have  settled,  into  a  series  of  vessels,  one  above  the  other. 
These  are  provided  with  double  bottoms.  The  two  uppermost  rows 
contain  cement  copper,  six  inches  deep;  the  lowest,  metallic  iron. 

The  fluid  deposits  its  silver  principally  in  the  first  tub,  dissolving 
at  the  same  time  an  equivalent  amount  of  copper.  Some  silver  which 


132  LIXIVIATION. 

escapes  precipitation  falls  with  the  cupreous  fluid  into  the  next  tub 
below,  where  the  rest  of  the  silver  is  taken  up  by  the  copper.  In  the 
third  vessel  the  copper  is  precipitated  by  the  iron.  The  brine,  freed 
from  silver  and  copper,  is  pumped  up  into  the  reservoir,  heated  and 
used  again.  The  cement  copper  obtained  in  the  last  tub  is  placed  back 
in  the  upper  two.  The  brine  circulates  in  the  tubs  until  a  bright  copper 
plate  is  not  coated  with  silver  when  held  in  the  fluid  from  the  leach- 
ing tubs.  The  residue,  which  is  mostly  copper-oxide,  is  removed,  and 
an  average  sample  taken  and  assayed.  If  it  should  contain  over 
eight  ounces  per  ton,  it  must  be  roasted  over  and  again  lixiviated. 

The  precipitated  silver  is  taken  out  once  a  week  and  treated  with 
muriatic  acid,  for  the  purpose  of  dissolving  the  copper  particles  which 
remained  with  the  silver.  After  this,  it  is  washed  with  water  till  all 
traces  of  the  acid  disappear,  then  pressed  into  balls,  dried  and  melted. 

Ziervogel  Process. 

Like  the  preceding,  Ziervogel's  extraction  of  silver  is  not  applied  to 
silver  ores,  but  only  to  copper  matte.  The  roasting  is  very  delicate, 
and  it  is  more  difficult  to  obtain  a  satisfactory  result -with  silver  ores 
than  by  a  chloridizing  roasting.  The  silver  in  this  process  is  converted 
into  a  sulphate,  which  is  soluble  in  water,  thus  dispensing  with  the 
expensive  salt  brine.  The  pulverized  and  properly  roasted  copper 
matte  is  charged  into  leaching  tubs,  500  pounds  in  each,  and  hot  water 
admitted.  As  soon  as  the  water  begins  to  flow  out,  the  hot  water  is 
made  a  little  acid  by  admixture  of  some  sulphuric  acid.  The  lixivia- 
tion  continues  until  a  sample  of  the  fluid  remains  clear  if  a  solution  of 
salt  is  added.  The  silver-holding  brine  is  conveyed  into  a  large  reser- 
voir, thirty  feet  long,  where  it  clears  of  impurities,  which  accidentally 
come  out  of  the  leaching  tubs,  and  falls  from  this  reservoir  through  a 
series  of  cocks  into  the  precipitating  tubs.  On  the  false  bottom  is  a 
layer  of  cement  copper,  and  upon  this  fifteen  to  twenty  copper  bars 
of  250  pounds  weight.  Each  is  fourteen  inches  long,  five  inches  wide 


LIXIVIATION.  133 

and  one  inch  thick.  The  liquid  loses  most  of  its  silver  in  these  tubs, 
and  flows  then  through  a  trough  fifteen  inches  wide,  lined  with  sheet 
lead  and  having  a  layer  of  copper  pieces  on  the  bottom,  into  five  vats 
filled  with  copper,  where  the  balance  of  the  silver  is  deposited. 

The  desilverized  brine  comes  now  into  a  reservoir,  whence  it  is 
pumped  up  into  a  large  leaden  pan  and  heated  again  by  means  of 
steam.  Above  this  pan  is  a  leaden  vessel,  out  of  which  about  thirty- 
drops  of  somewhat  diluted  sulphuric  acid  drop  into  the  liquid  every 
minute.  The  acid  prevents  the  separation  of  basic  salts.  The  silver 
is  taken  out  of  the  precipitating  tubs  every  day.  With  it  occur  some 
copper  and  gypsum.  The  larger  particles  of  copper  are  separated  by 
washing,  exposed  for  six  or  seven  days  to  leaching  with  diluted  sul- 
phuric acid,  and  finally  washed  with  hot  water.  The  silver  is  from 
860  to  870  fine.  After  drying,  it  is  refined  in  a  reverberatory 
furnace. 

Once  a  year  the  brine  is  brought  into  contact  with  iron,  in  order  to 
precipitate  the  copper.  The  purer  part  of  the  cement  copper  is  used 
for  the  silver  precipitation,  and  the  finer  part  is  delivered  for  smelting. 

Extraction  of  Silver  by  Sulphuric  Acid. 

This  method  of  extraction  has  not  been  applied  to  ores,  but  only  to 
copper  matte  and  copper  alloyed  with  silver.  The  result  is,  precipi- 
tated metallic  silver  and  sulphate  of  copper.  It  requires,  however,  a 
very  perfect  roasting,  otherwise  a  part  of  the  iron  contained  in  the 
matte  would  make  the  sulphate  of  copper  impure.  It  is,  therefore, 
more  proper  to  melt  the  matte  for  black  copper  and  to  treat  this  with 
sulphuric  acid  after  granulation.  The  granulated  copper  can  be  dis- 
solved in  wooden  tubs  in  diluted  boiling  sulphuric  acid ;  the  gold,  if 
present  in  the  copper,  remains  undissolved,  and  the  silver  is  precipi- 
tated by  copper  plates.  The  liquid  is  then  concentrated  by  evapora- 
tion to  the  crystallization  point.  A  better  economy  in  regard  to  sul- 
phuric acid  is  obtained  if  the  copper  is  first  oxidized  by  being  heated 
9 


134  LTXIVIATION. 

in  a  reverberatory  furnace  ;  but  then  it  is  generally  the  case  that  the 
copper  must  be  repeatedly  calcined,  two  or  three  times  before  all  is 
dissolved.  This  inconvenience  is  avoided  if,  as  before  stated,  the 
copper  is  subjected  to  boiling  in  diluted  sulphuric  acid. 

Dissolving  the  copper  in  concentrated  sulphuric  acid  is  less  advisa- 
able ;  it  requires  more  sulphuric  acid,  as  a  part  of  it  is  used  up  for 
oxidation  of  the  copper.  The  granulated  copper  is  treated  in  iron  or 
platina  vessels  with  strong  snlphuric  acid  of  64°.  A  great  deal  of 
sulphurous  acid  is  created  during  the  boiling,  and  this  can  be  utilized 
by  conveying  it  into  leaden  chambers  for  the  purpose  of  converting 
it  into  sulphuric  acid.  From  the  solution  the  silver  is  precipitated  by 
copper,  but  under  some  circumstances  it  may  be  more  advantageous 
to  precipitate  the  silver  by  sulphuretted  hydrogen,  which  precipitates 
first  the  silver  and  then  the  copper.  If  the  precipitation  is  stopped 
at  the  right  time,  sulphide  of  silver,  with  some  copper,  is  obtained, 
which  latter,  after  calcination,  can  be  separated  again  by  diluted  sul- 
phuric acid. 

The  Leaching  Process.* 

Under  this  name  is  understood  a  preparation  of  the  ore  applicable 
for  the  pan  amalgamation.  Its  description,  therefore,  does  not  belong 
here  strictly,  but  the  leaching  itself  has  so  close  a  connection  with  the 
preceding  manipulations,  that  this  part  alone  may  be  described  with- 
out mentioning  the  further  treatment  by  amalgamation. 

It  is  a  well  known  fact  that,  in  treating  refractory  ore  in  pans  by 
amalgamation,  of  course  by  way  of  roasting,  some  very  annoying 
things  are  encountered,  and  amongst  them,  principally,  the  great  loss 
of  quicksilver,  amounting  sometimes  up  to  ten  or  twelve  pounds  per 
ton  of  ore ;  the  rapid  destruction  of  pans,  which  compelled  many  mills 
to  use  wooden  sides  fixed  to  the  iron-pan  bottom,  a  measure  which 
saves  the  pans  at  the  expense  of  quicksilver;  and  the  very  base  bullion 
which  results  from  such  a  treatment.  In  some  instances  it  happens 

*The  Leaching  Process  is  patented,  as  ^n  application  for  pan  an4  barrel  amalgamation,  by 
G.  Kustel. 


LIXIVIATION.  135 

that  a  great  deal,  sometimes  over  fifty  per  cent.,  of  iron  goes  into  the 
amalgam,  rendering  the  continuation  of  the  amalgamation  impossible. 
The  result  of  the  amalgamation  of  base  metals  is  always  a  certain  loss 
of  silver  which  would  have  amalgamated  if  the  base  metals  were  out 
of  the  way.  It  happened  very  often  in  Nevada  that  $90  to  $100-ore 
was  purchased  for  the  purpose  of  amalgamating  it  in  pans;  but  a  few 
tons  proved  that  amalgamation  had  to  be  given  up.  Such  ore  is  now 
considered  suitable  only  for  smelting. 

At  a  very  trifling  expense  all  these  difficulties  can  be  avoided,  and 
the  amalgamation  turned  into  a  perfect  success;  for  instance,  the 
amalgamation  of  the  silver  ores  at  Flint,  Idaho,  turned  out  such  base 
amalgam  that  further  working  proved  to  be  ruinous.  The  introduc- 
tion of  the  leaching  process,  however,  resulted  in  a  most  favorable 
amalgamation.  It  is  only  to  be  regretted  that  after  working  several 
hundred  tons,  the  mine  refused  to  provide  the  mill  with  ore,  perhaps 
on  account  of  not  having  been  sufficiently  opened.  The  leaching  for 
the  pan  amalgamation  is  most  important,  and  at  the  same  time  cheap; 
all  the  expense  is  reduced  to  that  of  obtaining  hot  water.  This 
process  is  not  only  important  for  silver  ores  containing  base  metals, 
but  also  for  gold  ores  which  by  their  nature  require  roasting.  This 
refers  principally  to  auriferous  copper  ores,  as  the  amalgamation  of 
gold  is  very  much  obstructed  by  the  presence  of  copper  salts. 

It  is  a  matter  of  surprise  how  simple  a  remedy  could  have  been 
overlooked  while  fighting  with  the  obstructions,  caused  by  rebellious 
ores,  during  amalgamation.  If  there  is  soluble  chloride  of  silver  in 
the  roasted  ore,  and  besides  this,  soluble  chlorides  of  copper,  lead, 
antimony  and  zinc,  it  is  a  matter  of  course  that  all  will  be  decomposed 
and  amalgamated.  All  take  part  in  consuming  and  parting  the  quick- 
silver, and  in  destroying  the  pan,  hindering,  at  the  same  time,  the  easy 
amalgamation  of  the  silver  and  gold.  Why,  then,  not  put  all  these 
obstructive  metals  out  of  the  way,  and  give  the  silver  a  better  chance 


136  CHLORINATION. 

to  amalgamate?  The  base  metal  chlorides  are  soluble  in  water,  the 
chloride  of  silver  is  not.  It  is  therefore  a  most  simple  manipulation 
to  dissolve  those  salts  in  water,  and  to  remove  them  from  the  ore 
before  amalgamation,  by  the  leaching  process.  As  soon  as  this  is 
done  the  ore  is  divested  of  its  rebellious  nature,  and  it  behaves  in 
pans  like  the  best  ore.  The  process  of  leaching  is  described  (page  104.) 


IV.    EXTRACTION  OF  GOLD 

FROM  SULPHURETS,   ARSENIURETS    OR    QUARTZ  BY 
CHLORINATION. 


The  extraction  of  gold  without  quicksilver  is  limited  principally  to 
that  class  of  ores  in  which  the  gold  is  not  free;  that  is,  not  in 
metallic  condition,  but  combined  with  sulphur,  arsenic  or  tellurium. 
Ores  containing  free  gold,  finely  divided,  the  gangue  of  which  is 
quartz  without  admixture  of  sulphurets,  can  be  treated  by  chlorine 
gas  directly  without  roasting,  in  the  same  way  black  sand  may  be 
treated,  but  sulphurets,  arseniurets  or  tellurides  have  to  be  subjected 
to  a  thorough  roasting.  Silver  ores,  rich  in  gold,  can  be  also  advan- 
tageously treated  with  chlorine  gas  after  a  chloridizing  roasting, 
provided  that  no  coarse  gold  occurs  in  the  ore.  Ores  or  sulphu- 
rets containing  coarse  gold  are  not  suitable  for  chlorination.  By  this 
process,  if  properly  executed,  90  to  95  per  cent,  of  the  fire  assay  can 
be  extracted. 

In  order  to  be  sure  of  a  result  on  a  large  scale,  it  is  an  easy  matter 
to  make  an  experiment  with  twenty  or  thirty  pounds  of  sulphurets  or 


CHLORINATION. 


137 


of  ore  in  the  following  way:  The  named  quantity  must  be  roasted  first, 
and  it  is  the  most  difficult  task,  requiring  either  a  small  furnace  or  a 
great  deal  of  patience,  especially  when  small  charges  are  treated  on  a 
large  piece  of  sheet  iron,  having  a  charcoal  fire  beneath.  In  either 
case  the  sulphur  must  be  driven  out  perfectly,  so  that  when  in  a 
glowing  condition,  no  smell  of  sulphurous  acid  can  be  observed. 
When  finished,  a  sample  is  taken  for  an  assay,  and  the  roasted  stuff 
moistened  with  water,  after  the  weight  of  the  whole  has  been  noted. 

Fig.  17. 

f 


zu 

A  common  water  bucket  is  then  prepared  to  receive  the  moistened 
ore,  which  must  not  be  too  wet,  but  only  moist  enough  to  allow  its 
being  sifted.  On  the  bottom  of  the  bucket,  a,  Figure  17,  some  clean 
rock  or  broken  glass  is  placed  about  two  inches  deep,  and  covered 
with  a  piece  of  moistened  canvas.  A  short  glass  pipe,  c,  a  quarter  of 
an  inch  in  diameter,  is  inserted  close  above  the  bottom. 

The  ore,  d,  is  then  introduced,  filling  up  two-thirds  or  less  of  the 
space  as  loosely  as  possible,  and  covered  with  a  wooden  or  iron  cover 
and  pasted  all  around  with  dough.  The  cover  is  provided  with  a 
short  glass  tube,  like  c,  to  which  an  india  rubber  tube,  f,  for  carrying 
the  gas  out  of  the  room,  is  attached.  Both  glass  tubes,  c  and/,  must 
be  likewise  secured  with  dough. 

The  chlorine  gas  is  generated  in  a  glass  vessel,  A*.  There  are  two 
corks  in  it,  each  having  a  glass  tube,  as  represented  in  the  drawing. 
The  cork,  I,  is  removed  and  the  vessel  charged  with  3  ounces  of  pero- 


*  All  materials  necessary  for  such  an  apparatus  can  be  procured  from  John  Taylor  & 
:id  120  Market  street,  and  15  and  17  California  street. 


Co.,  118 


side  of  manganese,  4  ounces  of  common  salt,  and  4J  ounces  of  water, 
all  well  mixed.  The  cork  is  inserted  again  and  well  secured  with 
dough.  Another  vessel,  £,  provided  with  two  necks,  contains  water, 
as  indicated  by  g  ;  the  glass  tube,  h,  dips  about  one-half  inch  into  the 
water.  The  corks  are  made  air-tight  like  the  others  in  A.  The  whole 
apparatus  is  now  joined  together  by  rubber  pipe,  n  and  o,  fitting 
tightly  to  the  glass  tubes.  Having  all  thus  prepared,  7J  ounces  of 
sulphuric  acid  are  poured  through  the  safety-tube,  m,  but  only  in  small 
portions  and  at  intervals.  When  the  bubbling  of  the  water  at  g,  in 
the  vessel  B,  is  not  lively  enough,  some  more  acid  is  introduced,  and 
finally  the  temperature  raised  by  an  alcohol  lamp.  If  all  the  joints 
have  been  luted  carefully  with  dough,  not  the  slightest  inconvenience 
will  be  met  with.  The  chlorine  gas  from  the  generator,  A,  is  forced 
through  the  water  in  B,  by  this  means  washed  from  muriatic  acid. 
Through  the  pipe,  o,  it  enters  the  bucket  and  ascends  slowly  till  it 
reaches  the  cover,  escaping  then  through  the  rubber  pipe,  F,  where  it 
must  be  examined  from  time  to  time  by  dipping  a  glass  rod  into  am- 
monia and  holding  it  to  the  end  of  the  pipe,  x,  which  leads  out  of  the 
room.  In  contact  with  chlorine  the  ammonia  evolves  white  fumes, 
and  chlorine  can  be  detected  by  these  means  wherever  it  may  escape. 
The  gas  is  allowed  to  pass  through  the  bucket  as  long  as  chlorine  is 
created.  In  this  condition,  by  stopping  up  the  pipe,  x,  if  no  more 
chlorine  is  evolved,  the  apparatus  may  stand  until  the  next  day.  The 
cover  is  then  removed,  the  pipe,  o,  taken  off,  a  clean  glass  or  porcelain 
vessel,  as  indicated  by  z,  placed  below  c,  and  cold  water  carefully 
poured  over  the  ore  till  the  bucket  appears  to  be  full.  The  solution 
which  comes  out  at  c,  must  be  examined  at  times  in  a  small  tumbler 
with  a  few  drops  of  a  solution  of  sulphate  of  iron.  If  the  clear  solu- 
tion remains  unchanged,  without  becoming  darker,  the  lixiviation  is 
finished. 

To  the  solution  in  the  vessel,  z,    a  few  drops  of  muriatic  acid,  and 
then  sulphate  of  iron,  or  green  vitriol  (dissolved),  is  added  and  stirred 


CHLORINATION. 


139 


with  a  glass  rod.  The  whole  is  allowed  to  stand  till  all  the  gold  is 
precipitated  and  the  liquid  is  perfectly  clear.  This  is  drawn  off  by 
means  of  a  syphon,  for  which  the  rubber  pipe,  x,  can  be  used.  The 
remaining  fluid  and  the  precipitated  gold  are  gathered  on  a  filter, 
washed  with  warm  water  and  dried  with  the  filter  in  a  porcelain  cup, 
above  an  alcohol  lamp.  The  filter  is  burned  either  free  or  under  a 
muffle,  care  being  taken  not  to  lose  a  particle  of  the  filter  ashes ; 
mixed  with  some  lead  it  is  then  cupelled  and  the  gold  button  weighed. 
A  comparison  with  the  assay  shows  to  what  percentage  the  chloriiia- 
tion  has  proceeded. 

The  same  experiment  can  be  made  also  with  from  five  to  seven 
ounces  of  sulphurets  in  a  glass  cylinder,  or  other  shaped  bottle,  as 
Figure  18  shows,  the  bottom  of  which  is  cut  off  by  meajis  of  a  glowing 
piece  of  charcoal,  or  otherwise. 

FIG.  18.  The  roasting  may  be  performed  over  charcoal  or 

coke  in  a  small  stove  on  a  piece  of  sheet-iron,  the 
edges  of  which  are  bent  up  and  the  inside  coated 
over  several  times  with  clay-water,  and  then  well 
dried,  or  the  roasting  can  be  done  in  a  larger  black 
lead  crucible,  through  the  bottom  of  which  a  hole  is 
cut  for  the  draught.  The  sulphurets,  finely  pulver- 
ized, must  be  stirred  with  an  iron  spatula  until  no 
smell  of  sulphurous  acid  is  perceptible,  after  which 
a  strong  red  heat  is  applied.  "When  cold,  the  sample  must  be  ground 
over  in  an  iron  mortar  and  roasted  once  more  at  a  red  heat.  When 
cold,  it  is  moistened  in  a  dish  or  cup  with  sufficient  water,  to  make  it 
of  a  loose  or  wooly  consistency. 

If  the  roasting  is  perfect,  the  metallic  gold  could  be  dissolved  by 
leaching  with  chlorinated  water,  but  if  some  sulphurets  remain  un- 
decomposed,  it  is  more  proper  to  use  chlorine  gas.  The  pint  bottle, 
or  a  similar  glass  vessel,  is  prepared  with  a  cork  in  the  neck.  Through 


140  CHLOlUNATlOtf. 

the  cork  is  fixed  a  glass  tube,  to  which  a  rubber  pipe  is  attached 
through  which  the  -gas  is  admitted  and  which  serves  also  for  the  dis- 
charge of  the  solution.  The  neck  is  filled  with  fragments  of  clean 
quartz  or  broken  glass  covered  with  a  piece  of  cloth.  On  this  the 
moistened  ore  is  placed  and  proceeded  as  described  in  using  the 
bucket. 

Treatment  of  Sulphurets  by  Chlorination. 

This  process,  invented  by  Plattner,  was  first  successfully  introduced 
in  Silesia,  Germany,  in  the  year  1851,  on  auriferous  arsenical  residue, 
which  had  accumulated  for  many  years.  With  various  modifications 
the  process  was  practiced  afterwards  in  Saxony,  Hungary,  Transylva- 
nia, etc,,  and,  in  1858,  in  California.  This  process  is  very  perfect  if 
well  executed.  The  extraction  of  gold  from  roasted  sulphurets  by 
amalgamation  in  pans  is  very  difficult,  resulting  always  in  a  great  loss, 
while,  if  treated  by  chlorine,  the  same  kind  of  pyrites  will  yield  90 
per  cent,  of  the  gold  contained,  or  more. 

The  operations  to  which  concentrated  sulphurets  or  arseniurets  are 
subjected  are  principally  the  following  :  First,  the  oxidizing  or  chlori" 
dizing  roasting  as  explained,  (pages  21-52).  Next,  impregnation  of  the 
roasted  ore  with  chlorine  gas  in  wooden  vats,  then  leaching  of  the 
soluble  gold  with  cold  water,  and  finally  precipitation  of  the  chloride 
of  gold  by  sulphate  of  iron.  "  The  finer  the  gold  particles  are  the  easier 
and  quicker  the  chlorination  will  be  finished.  The  presence  of  copper, 
zinc  or  antimony  does  not  interfere  with  the  chlorination ;  lead  ore,  if 
too  much,  may  prevent  a  perfect  roasting,  and  silver  is  only  then  in- 
jurious if  alloyed  with  gold  in  too  large  a  proportion,  unless  it  is  very 
fine.  Generally,  the  silver  is  not  extracted,  for  the  reason  that  the 
amount  is  too  insignificant  to  admit  of  a  remunerative  extraction ;  but 
if,  besides  the  gold,  so  much  silver  should  occur  in  sulphurets  as  to 
make  it  an  object  of  saving,  it  can  be  leached  out  with  hyposulphite 
of  lime,  after  the  gold  has  been  removed^  and  it  will  be  found  that 


CHLORINATION.'  141 

this  silver  will  often  contain  so  much  gold  as  to  increase  the  value  of 
an  ounce  by  20  to  25  cents,  although  no  more  gold  could  be  leached 
out  with  water.  The  better  result,  obtained  by  O.  Hofmann's  pro- 
cess, extracting  the  silver  before  the  gold  is  chloridized,  is  described, 
page  122. 

Coarse  gold  requires  too  much  time  to  be  converted  into  a  chlo- 
ride. 

The  presence  of  lime  and  talk  (silicate  of  magnesia)  makes  the 
chlorination  of  roasted  ore  troublesome,  but  an  addition  of  one  to 
three  per  cent,  of  salt,  according  to  the  amount  of  those  earths, 
removes  the  difficulty;  but  for  all  that,  it  sometimes  happens  that,  in 
precipitating  the  gold,  other  substances  are  also  precipitated,  which 
will  be  mentioned  further  on. 

The  roasting  is  a  very  important  part  of  the  whole  operation;  on 
this  the  success  of  the  chlorination  principally  depends.  Concen- 
trated snlphurets,  if  there  is  no  opportunity  of  being  worked  directly, 
should  be  kept  under  water,  for,  if  exposed  to  a  slow  drying,  and  to 
getting  wet  alternatively  for  several  months,  it  forms  hard  lumps  and 
crusts,  and  must  be  pulverized  anew  before  charged  into  the  furnace. 

The  construction  of  the  furnace  has  but  little  influence  on  the 
result  of  roasting.  A  perfect  desulphurization  is  the  first  condition. 
This  requires  time;  and  using  a  reverberatory  furnace,  the  time  can  be 
given  ad  libitum,  10  or  30  hours,  as  the  case  may  require;  the  same 
refers  to  the  cylinder  furnace,  roasting  by  charges.  Furnaces  like  a 
retort,  or  a  muffle  furnace,  in  which  the  sulphurets  are  exposed  to  the 
oxygen  of  the  air  without  coming  in  contact  with  the  flame,  give  no 
better  results,  and  the  roasting  is  more  expensive  on  account  of  the 
consumption  of  more  fuel. 

In  regard  to  the  loss  of  the  gold,  it  has  been  stated  that,  in  some 
places  the  loss  amounted  to  a  very  high  percentage,  even  to  nearly 
one  hundred  per  cent.;  this,  however,  was  not  confirmed  by  the  ex- 
periments of  Plattner,  who  exposed  diverse  combinations  of  gold 


142  CHLORINATION. 

with  sulphur  and  arsenic,  different  ores  with  and  without  free  gold 
in  the  finest  state,  to  different  degrees  of  heat  and  time  of  roasting, 
and  the  result  showed  that  a  loss  of  gold  occured  only  when  the  roast- 
ing was  performed  so  rapidly  that  the  finest  gold  particles  were  carried 
off  by  the  volatile  products  of  roasting.  Many  other  experiments 
showed  that  the  loss  of  gold  is  inconsiderable.  The  addition  of  salt 
to  the  sulphurets  or  arseniurets  has  no  influence  on  the  loss  of  gold 
either. 

Application  of  Chlorine  Gas. 

After  the  ore  has  been  properly  roasted  (page  56),  it  must  be  mois- 
tened before  introduced  into  the  chlorinating  vat.  The  sulphurets,  if 
dry,  form  a  more  condensed  mass  than  in  a  certain  damp  condition, 
and  are,  therefore,  more  obstructive  to  the  ascending  gas;  besides,  the 
chlorine  does  not  attack  the  dry  ore  as  vigorouslv  as  when  it  is  damp. 
It  is,  therefore,  indispensable  to  moisten  the  sulphurets  after  they  are 
sufficiently  cool.  For  this  purpose  the  roasted  charge,  or  several  tons, 
are  spread  011  the  floor  and  water  sprinkled  over  it  by  means  of  a 
hose,  or  otherwise,  and  turned  over  several  times  till  it  appears  uni- 
formly wetted.  The  moistened  charge  should  not  create  the  slightest 
dust,  and  at  the  same  time  the  hand  should  remain  dry  on  handling 
it.  A  handful  of  it  pressed  hard  together  must  form  a  lump,  which 
can  be  held  with  the  fingers,  but  falls  into  its  former  loose  condition 
if  handled.  This  moistened  ore  always  contains  some  lumps  formed 
in  the  furnace ;  sometimes  it  may  happen  that  a  nail  or  screw  is 
charged  with  the  ore.  Iron  is  very  injurious,  if  in  the  ore  during  the 
chlorination,  and  the  lumps  would  not  chlgridize  properly.  For  this 
reason,  sifting  of  the  moistened  ore  is  necessary.  The  separation  of 
lumps  and  other  impurities  is  not  the  only  reason  why  sifting  should 
be  done.  The  more  chlorine  gas  that  can  be  introduced  into  the  ore- 
box  or  vat,  the  surer  will  the  chlorination  of  the  gold  be  effected ;  it 
is,  therefore,  important  to  sift  the  ore  directly  into  the  vat  in  order 


CritORINATION.  1  i3 

to  have  it  therein  in  as  loose  a  condition  as  possible,  leaving  more 
space  between  the  ore  particles  for  the  chlorine  gas.  The  sieve,  about 
eight  meshes  to  the  running  inch,  is  twelve  to  fourteen  inches  by 
twenty-five  in  the  clear,  the  sides  five  inches  high. 

The  chlorination  vat,  into  which  the  ore  is  sifted,  by  pushing  the 
sieve  to  and  fro,  either  on  two  scantlings  laid  over  the  rim  of  the  vat 
or  suspended  on  four  ropes,  is  seven  to  ten  feet  in  diameter,  capable 
of  holding  four  to  six  tons.  These  vats,  or  square  boxes,  are  provided 
with  false  bottoms,  in  order  to  permit  leaching  and  to  allow  the  gas 
to  expand  below  the  whole  mass  of  ore,  and  to  ascend  through  the 
ore  uniformly  at  all  points.  The  arrangement  of  the  filter  bottom 
can  be  fixed  in  the  same  way  in  a  vat,  as  shown  in  a  square  box, 
Figure  14,  page  105  ;  if  preferred  a  cotton  cloth  for  a  filter,  but  it  is 
more  expensive  than  a  gravel  filter,  the  cloth  being  soon  destroyed  by 
the  chlorine.  In  case  cloth  is  used,  it  is  necessary  to  lay  staves  across 
over  the  cloth  four  inches  apart,  so  that  when  the  tailings  are  taken 
out  by  means  of  shovels,  the  cloth  should  not  be  reached  and  dis 
placed. 


Fig.  19. 


Figure  19  represents  a  tub  filled  with  ore  on  a  gravel  bottom. 
Over  the  filter  bottom,  which  is  perforated  with  inch  holes  (or  direct 
on  the  bottom  of  the  tub),  is  spread,  first  a  layer  of  clean  quartz,  one 


144  CHLORINATION. 

and  a  half  to  two  inches  square.  In  default  of  quartz,  another  kind 
of  rock  or  gravel  will  answer  the  purpose,  except  lime  rock,  which 
absorbs  a  great  deal  of  chlorine.  Over  the  coarse  layer  smaller  pieces 
are  laid,  and  so  on,  decreasing  in  size  till  a  layer  of  sand  covers  the 
whole,  forming  thus  a  filter  of  from  four  to  five  inches  in  thickness. 
This  filter  remains  always  in  the  vat ;  the  shoveling  out  of  the  residue, 
therefore,  must  be  done  carefully  on  approaching  the  filter  bottom. 
There  are  two  holes  communicating  with  the  space  below  the  filter 
bottom.  One  is  for  the  reception  of  the  lead  pipe,  d,  or  better,  a 
rubber  hose,  by  which  the  chlorine  is  introduced ;  the  other,  c,  is  pro- 
vided with  a  hose  for  the  discharge  of  the  lixiviation.  This  side  of 
the  tub  stands  half  an  inch  lower  than  at  d. 

To  prevent  the  absorbtion  of  the  gold  solution  by  the  wood,  it  is 
necessary  either  to  keep  the  vat  filled  with  salt  brine  for  several  days, 
to  fill  up  the  pores  of  the  wood,  or  to  coat  the  inside  of  the  vat  with 
asphaltum  varnish,  or  a  mixture  of  pitch  and  tar  or  oil.  The  pitch 
mixture  must  be  applied  while  hot;  the  asphaltum  varnish  is  prefer- 
able, as  it  penetrates  the  pores  more  perfectly  than  the  pitch.  In 
either  case,  however,  after  a  long  use,  when  the  vat  "should  be  dis- 
carded, the  staves  should  be  burned  and  the  ashes  assayed  for  gold. 

The  moistened  ore  is  then  sifted  into  the  vat,  within  about  six 
inches  of  the  rim,  as  seen  in  the  drawing,  leveled  carefully,  without 
pressing.  The  wooden  cover,  a,  is  laid  upon  the  rim,  after  a  piece  of 
cloth,  b,  has  been  tacked  to  the  side  of  the  vat  near  the  water-pipe,  I, 
to  prevent  the  jet  of  water  disturbing  the  surface  of  the  ore.  The 
pipes,  i  and  I,  before  the  hose  is  used,  must  be  closed  with  stoppers, 
also  the  hose,  e.  It  is  better,  in  place  of  the  pipe,  I,  to  have  only  a 
hole  into  which  the  hose  is  inserted,  when  the  water  is  needed.  The 
chlorine  gas  can  be  admitted  now  into  the  impregnating  vat. 

At  Melrose,  six  square  boxes,  six  by  five  feet,  prepared  on  journals, 
admitted  a  very  convenient  and  quick  discharge. 


CHLOEINATION. 


145 


Fig.  20, 


Fig.  20  shows  a  vat  that  was  planned  by  E.  Eiotte,  for  chlorina- 
tion  works  in  Japan.  Vats  of  a  similar  shape  were  also  used  in 
Germany  to  chloridize  the  sand  in  the  river  Rhine.  It  is  a  deep  vat, 
but  since  the  roasted  sulphurets  filter  always  very  readily,  a  column  of 
sulphurets  four  or  five  feet  deep  still  permits  a  good  leaching.  The 
vat  measures  four  feet  on  top  and  two  feet  at  the  filter  bottom,  and 
is  five  feet  deep.  This  arrangement  has  the  great  advantage  of  taking 
up  less  room,  and  having  comparatively  a  much  smaller  fitlteing  sur- 
face, the  more  convenient  cotton  cloth  can  be  laid  over  the  false 
bottom,  which  is  perforated.  But  also  a  gravel  filter  between  the 
bottom  and  perforated  partition  can  be  prepared,  as  described  in 
Fig.  19,  which  does  not  get  out  of  order  by  being  turned  over.  The  gas 
is  admitted  through  the  lead  pipe,  a,  provided  with  a  cock,  by  which 
the  pipe  is  closed,  after  the  required  amount  of  gas  is  passed  into  the 


U6 


CHLORINATION. 


tub.  The  same  pipe,  with  another  hose  attached,  serves  for  the  dis- 
charge of  the  lixivium.  The  cover  is  represented  in  the  drawing, 
after  O.  Hofmann's  design,  This  arrangement  makes  the  cover  per- 


CHLORINATION.  147 

fectly  air  tight.  After  the  joint  on  the  periphery  has  been  carefully 
luted  with  a  good  plastic  clay,  also  around  the  two  pipes,  the  whole 
cover  is  then  covered  with  water  about  one  inch  deep.  The  clay, 
under  water,  of  course  cannot  crack,  and  this  is  an  advantage,  especially 
then,  if  before  leaching  it  were  found  at  one  of  the  pipes,  that  no 
surplus  chlorine  gas  is  in  the  tub  and  a  second  charge  of  gas  is  re- 
quired. 

This  gas  (page  14)  is  produced  in  a  leaden  vessel,  as  shown  in  Fig. 
21.  The  circular  tub,  a,  has  on  the  outside  a  circular  ring,  6,  two 
and  a  half  inches  deep  and  two  inches  wide,  for  the  reception  of  the 
flange  of  the  cover,  c.  The  cover  is  provided  with  a  center  hole  for  the 
reception  of  the  stirrer,  d,  with  a  leaden  pipe,  e,  bent  in  the  shape  of 
the  letter  02  through  which  the  sulphuric  acid  is  introduced  ;  another 
pipe,  f,  conveys  the  gas  to  the  washing  apparatus,  g.  At  the  bottom 
of  the  gas  generator  is  an  opening,  h,  for  the  discharge  of  the  residue, 
and  k  serves  for  introducing  the  charge. 

The  cover  is  placed  over  the  generator,  the  edge  of  the  flange  luted 
with  clay,  as  shown  by  m,  all  round,  and  then  filled  with  water,  as 
indicated  by  the  dotted  line.  The  space  at  n,  round  the  stem  of  the 
stirrer,  must  be  filled  with  packing  of  cotton  cloth  strips  saturated 
with  oil,*  and  the  outlet,  h,  closed  with  a  wooden  plug  wrapped  like 
the  stuff-box  in  cotton  cloth,  also  saturated  with  oil,  and  the  space 
between  collar  and  plug  luted  with  clay,  or  better,  with  putty.  The 
stirrer  rests  on  a  glass  button,  inserted  in  the  cavity  of  the  lead 
socket. 

All  roasted  sulphurets  do  not  consume  the  same  amount  of  chlorine 
gas.  The  following  approximative  charge  may  serve  to  chloridize 
three  tons  : 

*  It  is  for  the  manipulation  with  chlorine  gas,  a  very  important  discovery,  made  by  E.  Riotte, 
that  oil,  in  contact  with  chlorine,  forms  a  sticky  substance,  which  prevents  the  chlorine  from 
penetrating  it ;  and  the  india  rubber  hose,  if  oiled  inside  before  use,  retains  its  flexibility, 
especially  if  the  oiling  is  repeated  from  time  to  time.  It  is  thus  very  easy  to  stop  the  escape  of 
chlorine  gas  wherever  leakage  is  observed,  by  applying  some  suitable  material,  impregnated 
with  oil.  It  would  also  give  an  excellent  coating,  a  substitute  for  pitch  or  asphaltum,  if  the 
chlorination  vats  were  coated  two  or  three  times  with  warm  oil,  which  enters  readily  the  porea 
of  the  lumber  much  deeper  than  asphaltum  varnish. 


148  CHLORINATION. 

30  Ibs.  of  manganese  (peroxide)  pulverized, 
30  to  40    "    "  common  salt,  according  to  quality, 
75  '"    "  sulphuric  acid  of  66°  B.,  and 
45    "    "  water ;  or, 

30  Bbs.  of  manganese, 

30    "    "  salt,  good  quality, 

85    "    "  sulphuric  acid  63°  B.,  and 

35    "    "  water. 

Sulphuric  acid  of  66°  costs  4  cents  per  pound  in  carboys,  and  the 
same  of  63°  2 \  cents ;  the  latter  charge,  therefore,  is  cheaper,  unless 
the  freight  would  exceed  seven  cents  per  pound. 

Water,  salt  and  manganese  are  first  introduced,  and  the  charge 
hole,  k,  closed  with  a  wrapped  solid  plug  of  lead,  as  above  described, 
and  luted  with  clay.  The  generator  is  placed  in  a  square  piece  of 
boiler -iron,  the  edges  of  which  are  bent  up  two  or  three  inches  to  hold 
water  or  prepared  for  steam.  The  sulphuric  acid  is  now  introduced 
through  the  pipe,  e,  but  not  the  whole  amount  at  once ;  about  twelve 
pounds  to  begin  with  will  answer  to  create  sufficient  head  to  develop 
the  gas,  which  escapes  through /into  the  carboy  g,  to  be  washed  from 
acids.  The  gas  should  bubble  through  the  water  very  lively;  if  not, 
the  stirrer  should  be  turned,  and  if  this  fails  to  produce  the  required 
effect,  more  sulphuric  acid  must  be  introduced  and  repeated  as  often 
as  the  bubbling  appears  to  become  less  active,  always  using  the  stirrer 
first.  After  all  the  sulphuric  acid  has  been  used  up,  the  fire  must  be 
started  till  the  water  below  the  generator  commences  to  boil.  It 
takes  about  four  hours  to  develop  all  the  chlorine  gas. 

The  rubber  hose  from  the  pipe,  /,  (  J  inch  hose),  after  being  oiled 
inside,  is  inserted  into  the  carboy  one  or  two  inches  below  the  water ; 
the  other  hose,  ra,  conveys  the  gas  to  the  chlorinating  vat.  Both 
rubber  pipes  in  the  neck  of  the  carboy  are  wrapped  in  oiled  cotton 
cloth,  and  all  the  space  around  packed  tight  with  oiled  rags,  then 


CHLORINATION.  149 

covered  with  putty,  as  shown  in  the  drawing.  There  is  no  necessity 
to  change  the  water  in  the  carboy  often — once  every  month  will  do. 
The  water  through  which  the  gas  passes  absorbs,  if  cold,  about  two 
and  a  half  times  its  volume  of  gas,  and  is  then  saturated,  but  is  still 
good  for  taking  up  muriatic  acid. 

The  washing  of  the  gas,  to  get  rid  of  the  small  quantity  of  muriatic 
acid,  is  not  so  important,  since,  if  a  portion  of  this  should  happen  to 
enter  the  ore,  and  forming  sulphureted  hydrogen,  precipitate  metallic 
gold  of  that  already  chloridized,  this  would  again  be  converted  into  a 
chloride  in  presence  of  abundant  chlorine ;  but  the  washing  is  an  indis- 
pensable -indicator  of  the  progress  in  the  generator,  by  the  more  or  less 
lively  bubbling.  Care  must  be  taken  to  turn  the  stirrer  from  time  to 
time,  while  the  water  round  the  generator  is  boiling,  to  prevent  the 
caking  of  the  ingredients. 

If  there  is  only  one  chlorinating  vat,  the  gas  from  the  carboy  can  be 
conveyed  through  one  hose  direct  into  the  vat  below  the  filtering 
bottom ;  but  if  there  are  several  of  these,  it  is  better  to  fix  a  lead 
(|-inch)  along  the  vats,  connected  with  those  half -inch  pipes  before 
each  vat,  to  which  a  piece  of  hose  is  attached,  as  seen  in  Fig.  19. 
If  not  in  use,  the  hose  is  withdrawn  from  under  the  filter,  and 
closed  with  a  stopper. 

After  the  hose,  d,  has  been  inserted  under  the  false  bottom,  the  gas 
enters  the  space  and  rises  slowly  through  the  ore,  until  it  reaches  the 
cover,  and  commences  to  escape  through  the  pipe,  i,  or  in  Fig.  20, 
through  6.  This  is  easily  detected  by  the  smell,  but  it  frequently 
happens  that  although  the  smell  denotes  the  escape  of  gas,  it  is  some- 
times difficult  to  find  the  point  where  it  escapes.  Ammonia,  in  con- 
tact with  the  smallest  quantity  of  chlorine,  forms  a  white  fume,  and 
it  is  the  best  medium  to  examine  all  connections  therewith  *fco  find  the 
escape  of  gas.  If,  in  the  generator,*  little  gas  is  still  emitted,  this 

*  At  Melrose,  a  gas  generator  of  a  different  arrangement  was  successfully  in  use.  In  shape 
and  construction,  it  was  exactly  made  like  the  carbonic  acid  generators  for  the  fabrication  of 
soda  water.  The  cylinder  is  cast  lead,  and  has  for  protection  several  rings  of  boiler  iron  on  the 
outside  ;  only  pure  lead  can  be  used  as  solder. 


150  CHLORINATION. 

may  continue;  also,  if  all  outlets  in  the  chlorinating  vat  are  closed. 
After  this,  water  is  admitted  into  the  generator  through  the  pipe,  ft 
Fig.  21,  to  fill  up  the  space  inside,  and  to  displace  the  gas.  The  plug, 
A,  is  then  removed  and  the  residue  discharged  into  a  closed  trough. 

The  hose,  d,  in  figure  19,  is  also  removed  and  both  closed;  the 
hose,  c,  and  the  hole  at  d,  the  latter  well  luted  with  putty  or  clay. 
In  figure  20,  the  connection  is  cut  off  by  the  cock,  a.  In  case  the 
roasting  is  not  properly  done,  it  takes  several  pounds  more  of  man- 
ganese and  the  other  materials  in  proportion,  as  well  as  more  time 
for  impregnation. 

Lixiviation. 

After  twelve  hours,  or  if  the  sulphurets  contain  coarse  gold,  after 
fifteen  or  eighteen  hours,  the  little  cover,  r,  is  taken  off  to  see  whether 
there  is  enough  of  surplus  gas  in  the  vat;  if  only  very  little  is 
observed,  the  result  of  chlorination  is  not  certain,  but  if  all  gas  was 
consumed,  it  is  best  to  start  the  generator  and  impregnate  anew. 
Holding  before  the  opening,  r,  a  few  drops  of  ammonia  on  a  rag,  it 
must  show  a  thick  white  fume.  The  opening  is  closed  again,  and  the 
hose,  i,  is  led  either  out  of  the  building,  or  in  another  vat  already 
prepared  for  impregnation.  The  stopper  of  the  hose,  c,  is  removed, 
and  then  water  let  in  through  I.  The  upper  end  of  the  hose,  e,  is 
inserted  through  a  hole,  n,  below  the  cover  into  the  box,  to  let  the 
gas  escape  through  the  pipe,  i.  The  water  should  flow  in  quickly, 
until  it  nearly  reaches  the  hole,  n.  It  is  a  great  advantage  to  use  at 
least  one  inch  hose  for  the  water.  The  hose,  e,  is  now  lowered  into  a 
gutter  leading  to  the  precipitating  vat,  which  may  be  made  like 
the  vat  for  precipitating  the  silver  (page  114,  figure  15);  but  not 
higher  than  three  or  four  feet,  and  well  coated  with  asphaltum 
varnish,  or  it  may  be  made  as  a  rectangular  box  three  feet  deep,  and 
lined  with  sheet-lead,  half  round  on  the  bottom,  which  permits  an 
easier  and  better  cleansing.  A  very  smooth  surface  in  the  precipitat- 


CHLORINATION.  151 

ing  vat  is  important,  to  facilitate  gathering  the  finely  precipitated 
gold.  There  should  always  be  three  to  four  of  those  precipitating  vats, 
because  the  water  sometimes  shows  signs  of  gold  after  one  or  two  of 
the  precipitating  vats  are  already  filled. 

As  the  filtering  water  flows  from  the  gutter  into  the  precipitating 
tub,  samples  should  be  taken  frequently  in  a  clean  tumbler  of  white 
glass,  and  observed  whether  a  few  drops  of  sulphate  of  iron  (green 
vitriol)  cause  a  dark,  or  bluish  precipitation.  If  this  be  the  case  the 
leaching  must  continue  till  a  sample  under  such  examination  remains 
perfectly  clear.  After  this,  the  water  supply  in  the  chlorinating  vat 
must  be  stopped  and  all  the  liquid  contents  of  the  vat  permitted  to 
flow  into  the  precipitating  vat. 

4 

Precipitation. 

The  precipitant  for  gold  is  a  solution  of  sulphate  of  iron.  It  is 
known  also  by  the  name  of  copperas,  or  green  vitriol,  and  forms  light 
green  crystals.  Dissolved  in  water  it  generally  makes  a  muddy  solu- 
tion, but  it  clears  in  a  few  hours,  while  a  sediment  falls  to  the 
bottom.  It  is,  however,  better  economy  to  prepare  the  sulphate  in 
the  chlorination  works,  by  placing  old  iron  into  a  tub,  with  from  five 
to  six  buckets  of  water,  and  fifteen  or  twenty  pounds  of  sulphuric  acid 
in  two  or  three  charges  at  intervals  of  two  or  three  hours.  There  must 
be  always  a  surplus  of  old  iron  in  the  tub.  This  is  prepared  two  or 
three  days  before  the  solution  is  wanted.  One  bucket  of  this  solu- 
tion, or  less,  according  to  the  richness  of  the  ore,  is  poured  into  the 
precipitating  tub,  and  the  liquid  stirred  well.  The  precipitated  gold 
shows  a  red  brown  color,  but  there  may  be  a  great  deal  of  gold  yet 
in  the  liquid,  not  precpitated.  Whether  more  of  the  sulphate  is  re- 
quired can  be  ascertained  by  taking  a  sample  of  the  liquid,  filtering 
it  through  filtering  paper,  and  mixing  it  with  several  drops  of  the 
sulphate  of  iron  solution.  If  the  mixture  should  darken  a 


152  CHLORINATION. 

little,  after  a  few  minutes,  it  would  show  that  more  of  the  precipi- 
tant is  needed.  The  precipitated  gold  requires  some  time,  from  12  to 
1 5  hours,  before  it  is  deposited  on  the  bottom.  The  fluid  above  the 
gold  must  appear  perfectly  clear  before  it  can  be  drawn  off. 

The  water  which  is  drawn  off,  although  apparently  perfectly  clear, 
still  it  may  have  suspended  some  gold ;  it  is  therefore  very  important 
to  convey  it  into  a  large  tub,  where  it  is  allowed  to  remain  till  the 
next  charge  is  drawn.  From  this  vat  the  water  must  flow  through 
two  other  vats  with  filtering  bottoms,  which  are  covered  with  three 
or  four  layers  of  cotton  cloth.  Also  sawdust,  or  from  six  to  eight 
inches  tailings  from  roasted  sulphuret  tailings,  are  used  as  filters  for 
this  purpose.  After  several  months  of  operation  these  filters  may  be 
examined,  and  if  rich  enough,  the  gold  extracted  by  chlorination  with 
gas  or  leached  with  chlorine  water  and  precipitated  with  iron  solution. 

The  precipitated  gold  in  the  precipitating  vat  is  not  taken  out  before 
it  is  accumulated  from  a  longer  run;  it  is  more  profitable  to  make  as 
long  a  run  as  possible,  as  there  is  a  smaller  percentage  of  loss  by 
waste  in  handling  gold  in  large  quantities.  This  is  dipped  out  care- 
fully by  means  of  a  dipper  or  scoop  into  a  clean  porcelain  dish  or 
enameled  vessel,  and  then  the  tub  washed  clean  with  a  brush  and  a 
jet  of  water  on  the  sides  and  bottom.  The  gold  and  washing  is  then 
introduced  into  a  filter  of  filtering  paper,  or  of  cotton  cloth,  washed 
well  with  hot  water,  dried,  and  finally  fused  in  a  black  lead  crucible, 
with  addition  of  borax  and  some  saltpetre. 

If  the  sulphurets  are  pure  iron  sulphurets,  or  iron  pyrites,  with 
some  quartz,  the  precipitated  gold  is  of  a  fine  red  brown  color,  filters 
and  fuses  easily  without  any  trouble,  but  it  is  different  with  impure 
sulphurets.  Of  these  the  leaching  after  chlorination  appears  clear  and 
of  the  proper  yellow  color,  but  on  addition  of  sulphate  of  iron  the  pre- 
cipitation often  appears  of  a  blackish,  bluish,  or  whitish  color  ;  the 
precipitated  gold  is  voluminous,  it  filters  very  slowly,  and  in  smelting 


CHLORINATION.  153 

a  great  deal  of  borax  must  be  added.  The  impurities  are  mostly 
iron,  sulphate  of  lime,  etc.  A  great  deal  of  these  impurities  can  be 
dissolved  by  pouring  from  five  to  ten  pounds  of  sulphuric  acid  into 
the  precipitating  tub  after  the  clear  water  has  been  removed,  but  it  is 
much  better  to  boil  the  gold  in  an  enameled  kettle  with  the  acid, 
then  dilute  it  and  filter.  The  gold  will  be  still  mixed  with  foreign 
matter.  In  many  similar  cases  a  better  result  is  obtained  if,  imme- 
diately after  leaching,  sulphuric  acid  is  added,  well  stirred,  and  the 
next  day  the  clear  liquid  drawn,  over  into  another  precipitating  vat 
and  the  gold  precipitated  with  sulphate  of  iron. 

In  Europe  sulphureted  hydrogen  is  used  to  precipitate  the  gold  as  a 
sulphide;  the  sulphureted  hydrogen  is  obtained  there  by  heating 
matte  from  lead,  or  smelting,  with  diluted  sulphuric  acid.  A  conve- 
nient and  cheap  way  to  produce  that  gas  is  to  heat  in  a  proper  glass 
vessel  parafnne  and  sulphur  flour.  The  precipitation  with  sulphate 
of  iron  is  preferable. 

The  cost  of  chlorination  and  the  roasting  in  a  long  furnace  of  two 
tons  of  sulphurets  per  twenty-four  hours,  calculated  on  San  Francisco 
prices  of  materials,  the  sulphuric  acid  in  carboys,  is  as  follows  : 

Superintendence $  6  00 

Two  roasters,  @  $2.50 -. 5  00 

Two  cords  of  wood,  @  $5 10  00 

20  Ibs.  of  peroxide  of  manganese,  @  $2.75 0  55 

27  Ibs.  of  salts  (ground),  @  50  cts 0  13£ 

57  Ibs.  of  sulphuric  acid  63°,  @  $2.50 1  42£ 

One  man,  at  the  rate  of  $2.50 2  50 

Sulphate  of  iron,  about 0  29 

Total $25  90 

Or  $13.00  per  ton  ot  sulphurets. 

The  presence  of  galena  necessitates  a  good  roasting,  and  a  strong 
finishing  heat,  as  far  as  the  fusibility  of  the  metal  permits  it.  Ore 
of  this  kind  requires  a  long  furnace  in  order  to  prepare  the  ore  for 
the  stronger  heat  while  it  progresses  from  one  hearth  to  the  other. 
In  case  the  ore  is  not  properly  finished,  the  undeconiposed  sulphurets 


154  CHLORINATION. 

and  sulphates  would  absorb  a  great  deal  of  chlorine.  An  experiment 
made  with  two  samples  of  ore,  the  one  contained  three  per  cent,  of 
galena,  the  other  thirty.  After  roasting  without  salt,  both  were  sub- 
jected to  impregnation  with  chlorine  gas.  The  first  ore  consumed 
the  usual  quantity  of  gas,  and  gave  85  per  cent,  of  the  gold  con- 
tained, but  the  tailings  showed  coarse  gold  particles  which  could  not 
be  chloridized  in  twelve  hours.  The  other  lot  absorbed  twice  as 
much  chlorine,  was  leached  after  24  hours,  and  the  result  was  62  per 
cent,  of  the  gold.  The  tailings  contained,  after  grinding,  specks  of 
undecomposed  galena,  which  proved  that  the  roasting  was  either  too 
short  or  there  was  not  sufficient  heat.  The  coarse  gold  was  taken  up 
instantly  by  quicksilver  in  consequence  of  the  clean  surface  created  by 
the  action  of  chlorine,  but  if  in  such  a  case  quicksilver  should  be  used, 
the  tailings  ought  to  be  washed  or  the  leaching  continued  till  all  chlo- 
rine disappears,  and  the  tailings  amalgamated  while  yet  fresh,  because 
if  too  long  exposed  to  the  action  of  the  air,  the  gold  particles  soon  ap- 
pear reddish  and  do  not  readily  amalgamate. 

The  question  in  regard  to  utilization  of  the  surplus  gas  which  re- 
mains in  the  vat  after  the  chlorination  has  been  finished,  seems  not 
to  be  solved  yet  by  practical  use.  A  vat  of  seven  feet  diameter  and 
two  feet  high,  when  filled  with  roasted  ore  to  within  six  inches  of  the 
top,  leaves  about  forty-seven  cubic  feet  of  space  for  the  gas.  This 
space  should  be  always  filled  with  chlorine  gas,  and  should  be  re- 
placed if,  before  leaching,  it  were  discovered  that  all  the-  gas  was  con- 
sumed on  account  of  improper  roasting,  or  from  some  other  cause. 
Having  then  two  impregnation  vats,  the  communication  between 
them,  for  the  purpose  of  conveying  the  gas  from  one  into  the  other, 
is  easily  prepared  by  a  hose  from  the  top  of  the  one  connecting  under 
the  filter  bottom  with  the  other  vat.  The  admitted  water  will  force 
the  gas  into  the  other  vat,  but  as  the  cold  water  dissolves  a  great  deal 
of  the  gas,  perhaps  more  than  the  half  of  the  whole  amount,  which 


CHLORINATION.  155 

for  the  extraction  of  the  gold  is  advantageous,  it  is  hardly  worth 
while  to  utilize  the  remainder  of  the  gas,  if  it  were  not  for  the  object 
of  getting  rid  of  it  in  an  inoffensive  manner. 

At  Schemnitz,  Hungary,  the  tailings  from  the  silver  extraction  by 
Ziervogel's  method,  are  treated  for  gold  by  the  chlorination  process. 
They  contain  about  three  ounces  and  a  half  of  gold  per  ton.  The 
gas  generator  is  made  of  cast  iron  with  a  leaden  hood,  joined  by 
flanges  and  bolts.  The  hood  has  three  openings,  for  charging  man- 
ganese, for  sulphuric  acid,  and  the  third  for  the  exit  of  the  gas.  This 
enters  first  the  washing  vessel,  then  a  receiver,  and  thence  passes 
through  "two  pipes  into  earthen  impregnation  vessels.  The  lower  part 
is  contracted  like  a  funnel,  and  receives  a  filter  of  quartz  some  two 
inches  deep.  The  upper  part  is  furnished  with  a  clay  cover  provided 
with  a  suction  pipe. 

The  tailings,  having  been  dried  and  slightly  glowed,  for  the  purpose 
of  destroying  the  basic  salts,  are  moistened  in  the  usual  way,  and  six 
hundred  pounds  introduced  into  each  of  the  four  chlorination  vessels. 
The  chlorine  is  then  introduced  through  a  pipe  below  the  quartz  filter 
and  continued  until  the  gas  is  distinctly  perceived  above  the  charge. 
The  cover  is  then  put  on,  and  luted  with  dough.  ^The  tailings  are  ex- 
posed to  the  action  of  the  chlorine  for  twelve  hours.  At  the  end  of 
this  time  warm  water  is  introduced,  and  the  lixivium  collected  in  large 
glass  bottles,  where  the  gold  is  precipitated  by  sulphate  of  iron. 

Cotvert's  Method  for  Auriferous  Quwrtz. — This  method  is  based  on 
the  production  of  chlorine  in  the  mass  of  ore.  It  is  cheap,  and  per- 
mits of  the  extraction  at  the  same  time  of  both  silver  and  copper,  and 
is  not  injurious  to  the  workmen.  The  finely  pulverized  quartz  is 
mixed  with  one  per  cent,  of  manganese,  charged  into  vats  and  muri- 
atic acid  added.  In  this  condition  the  ore  is  kept  closed  in  the  tubs 
for  twelve  hours.'  There  is  a  perforated  filter  bottom  in  the  tub  cov- 
ered with  some  brush  and  straw.  The  mass  is  then  leached  several 


156  CHLORINATION. 

times  with  the  same  water,  and  then  conveyed  into  precipitating  tubs, 
where  the  copper  is  first  obtained  by  means  of  iron,  and  afterward 
the  gold,  the  chlorine  having  been  driven  off  by  heating  the  liquid. 
The  precipitant  is  sulphate  of  iron. 

If  there  is  silver  in  the  ore,  the  chlorine  is  generated  with  salt, 
manganese  and  sulphuric  acid  taking  six  parts  of  salt  to  three  parts 
of  manganese.  The  formed  chloride  of  silver  dissolves  in  the  salt  so- 
lution and  is  precipitated  by  copper  plates,  the  copper  by  iron,  and 
the  gold  by  sulphate  of  iron. 


PACIFIC  IRON  WORKS 

RANKIN,   BRAYTON   &    CO., 

PROPRIETORS. 

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